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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old June 21st, 2017, 11:20 PM
ItalOso ItalOso is offline
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Default Manual to Automatic

Hello Members,
Has anyone ever changed their SM420 Muncie to an automatic on a 250-I6 c.i.? I've been told going to something like a 700-R4 is overkill because it needs a special linkage that, if not installed correctly, can burn up the tranny in a matter of miles.
One recommendation, on 3:07 rear end gears is to go to a turbo 350 and do the applicable driveline adjustments, amongst other things.

Would putting a shifter on the hump be easier than getting a new steering column? Leg room is a must as it is already, the more room would be nice. Although I want to remove the stock steering wheel because of it's size - '64 GMC - and go to a smaller wheel and tilt option.
Any info or help would be greatly appreciated. Thanks!
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  #2  
Old June 22nd, 2017, 08:20 PM
AZKen AZKen is online now
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Default Re: Manual to Automatic

Basically you need to know what you are doing to install any auto trans. Nothing is real technical but there are things that you need to do correctly. If you need the OD of a 700r4, the set up of the TV cable is straight forward if you follow instructions. If not, a 350 is plenty good. The yoke and length of the driveshaft will need modified. Lokar makes a floor shifter for either. Don't understand the comment about leg room. The floor shifter is not in the way and neither is a tilt column shift. The floor shifter is easier. You will lose a pedal. That's more leg room.
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Old June 22nd, 2017, 08:32 PM
ItalOso ItalOso is offline
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Default Re: Manual to Automatic

Thanks AZ Ken...my comment about the leg room is for my height, 6'6" but my grandfather bought this truck when I was born, family history...learned to drive in it.
I am trying to gain room not just side to side, but length/depth as well. I know the new tilt steering column would allow more room, a smaller steering wheel, and might as well get the proper column for the shifter...and lose the floor shifter as well.
I won't do the work myself, have a friend who's a mechanic, and may assist him to see and know the end result. Writing here gives me sense of what's needed and necessary.
Some members have mentioned that you can extend the cab, but that seems like it would involve a lot of cutting, fabrication, welding..and I'm not sure that's worth it.
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Old June 22nd, 2017, 08:41 PM
AZKen AZKen is online now
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Default Re: Manual to Automatic

I don't know how much room you would get but maybe putting a fuel tank under chassis and moving seat back may give more leg room. Extending cab would not be something I would consider. I'm sure this is not the only vehicle that you have a leg room problem with. It is also possible to modify the brake pedal closer to floor with just enough room to stop. Together with a shorter stroke power brake set up.
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Old June 23rd, 2017, 06:36 PM
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ilvracn ilvracn is offline
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Default Re: Manual to Automatic

You can gain several inches by relocating the fuel tank, if you put a seat out of an 88 to 99 chevy/ gmc full size truck in. The back is much thinner and also much more comfortable. I have a buddy that has a 60 panel truck and he loves the newer seat, he is about 6 foot five, said it gave him much more room
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Old June 24th, 2017, 01:42 AM
AZKen AZKen is online now
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Default Re: Manual to Automatic

A panel is supposed to have "bucket seats" not a bench seat. There is no bulkhead behind the seats. So just moving the seat back would have worked. Your friend may have extra room because he mounted the 88-99 way back. A pickup has very little room to move back. The stock 64 seat back can be re-foamed with high density foam/no springs and made a little thinner.

Last edited by AZKen; June 24th, 2017 at 01:48 AM.
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Old June 25th, 2017, 07:39 PM
ItalOso ItalOso is offline
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Default Re: Manual to Automatic

I have installed new front power disc brakes, but it didn't shorten the brake pedal...which would be nice.

I am looking for a resource that sells the "square or rectangular rubber bumpers" that the brake and clutch pedal hit, at rest, up near where the column mounts. They're worn out and need to be replaced. They would hold the pedal arms closer to the floor, giving me more room to lift and move my feet. Checked out LMC parts, but their picture shows a totally different design. Found a place that rebuilds the pedal assembly on Ebay, but didn't save page. Don't know the proper term for the parts.

Drove it yesterday to a shop to put it in the air. Going to install the new fuel tank between the frame in the rear by the bumper, with a side fill. Any recommendations for a new tank? Side fill door, square or round on the rear panel?

As for the bench seat, I measured the original seat to have about 20-22" on the base and 20" from the crease to the top of the seat. The base hits me about mid-thigh. The springs are shot because of my size. Lowering it would be good too, will talk to my upholsterer about dense foam & springs out completely. Will look at ilvracn's recommendation for that 88-99 seat. Stock is 58-59" wide.

In removing the tank, I measured about 4-5" more space that a new seat would be able to slide back...although as AZKen mentioned, the backrest would be quite vertical.
Under the small rear window, the cab flares inwards, "the bulkhead" I suppose, as somewhat of the backrest support. As anyone ever altered/cut this out for more vertical room? Or would the rear of the cab lose it's integrity to a point?

Does anyone know the stock height of the steering column? I don't have power steering and looking to replace to a tilt, with a smaller steering wheel. Need to make decision on replacing the SM-420 Muncie out and put a turbo 350 automatic in with the shifter on the column. May even go with a crate 350 engine too.
If I get the extended room with the seat project, I may keep the Muncie..trying to do too much at once!!! My rebuilt 250 runs well but I am more than cramped around the stock wheel and angle of my right foot on the accelerator.
Thanks for your help.
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  #8  
Old June 26th, 2017, 12:24 AM
AZKen AZKen is online now
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Default Re: Manual to Automatic

Well if you are thinking out of the box, I will go there with you. Your truck came in small hump floor and large hump floor. Automatics have small hump. If the center hump is an issue for you, you can patch in a small hump or any sheet metal you want if you use an automatic trans. You can cut the whole hump out all the way to firewall. Install trans and build a new floor around it if that helps.
When I said modify brake pedal I meant shortening or lengthening the arm to make it closer to the floor. Your idea to bump stop it may work also. Just use same bumper and/or any rubber/metal spacers to experiment and get it where you want. The brake pedal arm can be any shape, bent towards the left and back or whatever. Just so it still pushes the rod......or.....

Eliminate the brake pedal/arm and move accelerator pedal left. Modify the movement of the clutch pedal to operate the brake rod. I do see how to do that.

Some of these steps need to take place:
Take out the unused pedal, take out tank, reposition seat and see where you are at. Unbolt and move the seat rearward to mock this up.

The flared out sheet metal under the rear window can be modified. Keep the ends of the flare attached to cab wall as a gusset. (Don't cut them out) The cab wall is already dimpled for strength and partial removal of the "sill flare" should not cause any loss of structural integrity. The issue would be if the bulkhead just below the window would "oil can", it would pop the glass out. A channel or angle piece could be added across and be much thinner than the present flare. I think the flare shape is mostly to blend out to the tank and act as a "cosmetic return" for the seat back.
Have someone else with a mechanical engineering background look at it first. I have one and that's my take on it.

I guess it MAY be possible to add a deep footwell/firewall bump-out on drivers side. (don't quote me on that one!)

When you add a new tilt column, you can mount it at any angle and with any stick out that benefits you. These things need to be mocked up and tried together before final attachments. Try to think of a way to get rid of the knee knocker Ebrake lever.

Lots of options for a tank including 60-62 Panel and Suburban donor tank. They are under chassis and can be used on a truck. This and This and This are available also, but are for Chevy so check dimensions and clearance.


I'm starting to feel very weird about my suggestions, but you wanted ideas.

Last edited by AZKen; June 26th, 2017 at 06:52 AM.
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