[raycow]

Mel -

IMO, Steve's solution is workable, but it all depends on a major unknown: Finding a suitable rear end.

First, you need to determine your present ratio, so you know what you are starting with. This is easy enough to do by counting driveshaft revolutions. There is no need to take the cover off and count the teeth unless you want to. Next, you need to decide how much rpm reduction you want, as a %. Most overdrive transmissions run in the 25-30% range (0.70-0.75 O/D ratio), just to give you an idea of what is out there.

You probably have a H052 or H072 rear (3/4 and 1 ton, respectively). These two are very similar, with the main difference being wheel bearings. The tallest gear ever offered for these was 4.11, so I don't think you will be happy with another of the same type. GMC also used a Dana 60 rear for some 3/4 ton applications, and taller gears are available for this one at least down to 3.54.

In 1973, GM replaced the H052/72 with 2 versions of the 14 bolt rear, which may still be in use. I don't know the specific ratio applications, but they are definitely available in the 3.xx range. This is something you will have to research. The main problem with the 14 bolt is that all the ones I have measured are wider than the rear you have now. Assuming you want to keep the wheels in the same place, you will either have to get wheels with a different offset, or else narrow the axle housing (expen$ive).

Fortunately, other truck makes used the same 8 bolt wheel pattern that you have, so you can extend your search to include Ford and Dodge in the hope of finding one of suitable width.

Regardless of what rear you end up with (including later GM), it isn't likely to be a bolt-in, mainly because of spring pad spacing, so you will probably have to relocate the spring pads. You will also have to cobble the parking brake cable hookup unless you get a transmission with the parking brake on its output shaft. Fortunately, those are fairly common.

As for the SM465 transmission, it will almost bolt up, but not quite. You will have bore out the center hole in the bellhousing to accept the SM465 bearing retainer. The "correct" 4 speed for your truck would be a SM420, which will fit your bellhousing without needing any modification. The SM465 wasn't offered until 1968. Both transmissions are of similar strength, but the SM465 has better parts availability, particularly the 2nd gear synchronizer, which is getting hard to find for the SM420.

Rick's post got me thinking again about the SM318 and its GVW rating. It didn't seem reasonable that GM would sell a truck with a transmission which wasn't adequate for the maximum GVW of the truck it was installed in. I wasn't able to find the applicable GMC literature online, but I did look at some Chevy specs. The C-20, which is their 3/4 ton, is rated up to 7500 GVW, and it came with a SM318. Their next heavier series, the C-30, is rated up to 10,000 GVW and you couldn't get a SM318 in that one. If you wanted a 3 speed, you got a T-89, which is stronger.

So where I am going with this is that I was wrong about the 6000 GVW rating for the SM318. If you are at 7500 or less, GM claims it is strong enough for the job. This means that the SM318 with the B-W O/D should again be given consideration. It will definitely be an easier and less expensive installation than the SM465 plus a rear end swap. Btw, the B-W O/D can also be used as a gear splitter if you wire it for manual control, which is what I did in all the cars I installed one in. As I said in a previous post, I would have one in my truck now if it had come with a column shift.

Ray