The Stovebolt Site Forums The Shop Area Driveline 350(Stroker) and SM420 Combo in 59 Burb

Quick question for all. I have a 59 Burb coming together. I have a nice freshly rebuilt 350 (383 stroker) I'm installing. I still have the SM420 tranny. I'm going to be pulling a 19 ft Winnebago travel trailer. Will my stroker and SM420 combo will do me good?? The rear end is stock 390:1

Also, any experience with a modern (2006 or earlier) 5 speed truck (MW3) tranny behind a small block on a 55 - 59 task force?

Thanks for any input!

The SM-420 is a solid 4-speed that can handle the 383. It will give you granny gear when you really need to pull out of a hole with a heavy load and has a nice spread between all of the gear ratios. Final gear (high Gear) is 1:1 just like the TH-350/400 and everyother non-OD manual transmission. This trans was also used in the 2-1/2 ton trucks rated at 19,000 GVW. Under normal driving conditions you'll drive it like a 3-speed (2, 3 & 4).

Low gear is not synchronized, all others are...

Mike B

Maddoggee1959,

I don't think with the driveline combination with your freshly built 383 stroker will give the engine what it needs to reach it's full potential. What cam and heads are you running on the 383 stroker? I built a 383 stroker with a split duration Crane street/strip cam and reworked heads from Indy Head Service for my previously owned 1958 Chevrolet Biscayne 2 door post. The heads were designed to run with my cam specs. I ran the stock Muncie 3 speed with 3.55 posi rear end gears. The rear gear ratio was GREAT, but the stock 3 speed did not let the 383 reach it's full rev potential. I ended up rebuilding the 3 Muncie 3 speed with a "hotrod" kit from Driveline.com. The "hotrod" kit gearing gave first and second gear the correct ratio so that the 383 could reach it full power potential. If it were me I would look into a 4,5 or 6 speed transmission so that you can use 1st and second gear. The SM420 1st gear is a "granny gear" for low RPM and torque only. 2nd gear is what most people use to start from a stop on the street. The 3.90 gear is too high for highway driving. I would go with 3.72 or 3.55 if you want to cruise at 65-70 MPH. Just suggesting.

Tim

Thanks! Sounds like it's a functional set up. I assume engine mounting will be … front eng mounts and rear bell housing mounts? My 350 is from 71 Chevy Nova. Will the bell housing that is currently between the SM420 and my 235 work for the 350?

Hy Maddoggee1959, you will need a bellhousing from a V8 equipped truck they were cast iron and used between 1955B and 1966 I believe, hope that helps.

I had a 350 in my 59 Suburban in front of the SM420 for years, I replaced the rear with a 12-bolt in a 3.4 gear ratio, plenty of power in granny to get it moving with whatever is behind it, and 90 down the freeway if you so desire. I swapped the SM420 out for an SM465 because it had a reverse light switch and was in better shape (SM420 had a noisy input bearing)

I did a lot of towing with a 59 Burb with a completely stock 1970 307 2-barrel engine and a SM-420, including a 22-foot Norris camper that weighed in at close to twice what a modern RV of similar size would be. That thing was a real lead sled! No problems at all! However, I was using a Dana type 60 3/4 ton rear end with a 3.73 ratio and a limited slip carrier. Your 3.90 will be a pretty good towing gear, but I question its strength with the engine/trans setup you have. I'd recommend going to something like a 9" Ford rear axle, or something similar from a GM pickup a little newer than the original rear axle. A ratio somewhere in the 3.7:1 range will give you good road speed, and the granny gear of the SM-420 will handle getting moving on even a steep upgrade.
Jerry