The Stovebolt Site Forums The Shop Area Engine Shop swap sm420 4 speed for 3 speed?

I have the original sm420 4 speed trans still in my '49. Its behind a 250 inline 6 motor. I've been wanting to swap the trans for one a little better for cruising the highway and get the rpms lower. A buddy of mine has a 3 spd trans out of a '68 chevy pickup I can have pretty cheap but didn't know if it would really be worth doin. I do have a turbo 350 but didn't really care to go auto. If someone that knows about these can help me out that would be great!

The 3 speed and the automatic will not give you any gains over the 4 speed, as the final drive will still be 1:1. In order to get a better highway gear, you'll either need to change out the rear differential gears or go to an overdrive transmission, like the T5 or the 3 speed with the B/W overdrive.
I took a look at your pix, did you swap to an open driveline? If so, the T5 would be an easy swap, and would keep the floor shifter. Alternatively, there is also a 3.38 gearset available for that rearend. Call Patricks if you want to go that route.

No advantage to swapping unless you can find an overdrive transmission. (think T5)

The SM420 and most 3 speeds are direct drive in high gear.

Grigg

Patricks gears are 3.55. 3.38 was a possibility on a automatic, but you would have to go to an open drive shaft.

The drive shaft is half open and half closed, something my wife's grandfather must have come up with in his state of the art backyard dirt floor workshop so he could put a different rearend. Seems to work and doesn't vibrate. I've since swapped the rearend with a rearend with 3.90 gears. It helped a lot with the cruising but still just not enough. I guess I'll just go the auto route then since I already have a turbo 350. Thanks for the help guys!

It sounds like you are describing the drive train of a 3/4 Ton. They had a short tube followed by the open driveline.

That will not make any difference in speed or rpm, 3rd gear is still 1:1. You'll need an overdrive trans, like a T-5, SM318/326 with the Borg Warner overdrive, the early '80s MY6, or a 700R4 to get any gains.

The pictures you have is of a 3/4 ton. The original gears would have probably been 4.56. 3.90 was an option on 3/4 ton later and it was probably installed along with the later engine. The drive shaft you picture is a typical 3/4 ton. There were a couple of years that something like 3.38 was offered in a 3/4 ton auto with a big block engine, but they are extremely rare. How fast do you want to go? The 3.90 will go as fast as it is safe to go with stock steering and brakes.

I never thought that the auto would be the same but I guess it would if the final gear is 1:1. I'll have to start looking for something else.
The rearend that is under the truck now is out of a '58 1/2 ton truck that I installed. With the original rearend which had 4.56 gears, I was only able to go around 45-50 mph. With the newer rearend I can now go 55-60 but the engine is running right at 3000 rpm. I don't want to go much faster, just get the engine at a comfy cruising rpm.
I also want to swap the original trans because I cant ever seem to keep the clutch adjusted. I'll adjust it and it'll be good for a little while but then won't want to fully release.