Small Transmision Hump to Large hump

[kaneerarchery]

My 1966 GMC started as a 3 speed on the column truck with an inline 6. I found a 4 speed out of a 1966 that I would like to use. How much of a job is changing to a large hump setup if i do not have the portion of the large hump that is a part of the firewall. I could fabricate the part of the floorboard to raise the hump without much of an issue, but I do not know how much additional clearance is needed at the firewall. I plan to stitch weld the new panel in place.

Randy Kaneer

[BarryGMC]

You need this from the firewall. You cut one like this then remove the hump side at this seam that you can see on the back. Pretty easy really.

[BarryGMC]

Another pic.

[BarryGMC]

Well I just realized you are in Florida. You may have trouble finding that part. If so let me know. Barry

[kaneerarchery]

Thanks Barry. Great photos. I just ran across an article (by Craig Ueltzen, Edited by Jolly) titled "Upgrading a SM-318 3-speed to a SM-420 4-speed" on 6066gmcguy.com. I will let you Know if i have trouble finding a donor firewall section of the large hump.
Thanks. Randy Kaneer

[kaneerarchery]

Will the NP833 4 speed O/D transmission from 80-87 bolt up to an inline 6 cylinder and fit under the small transmission hump in my 1966 GMC.
Thanks.
Randy Kaneer

[BarryGMC]

Ok . Yes it will bolt to a 230-250-292. With a 67-90 bell housing. Yes it will fit under the hump. But you need a tranny crossmember. The bell housings don't have side mounts. And you need to shim the crossmember down about a 1/2 inch if the cab mounts are old. It's a little tight if the mounts are old and sagged. It's a good setup even though Chrysler built it. BC