I can tell you that the New Process 540 series of transmissions are not a bolt up replacement for the SM-420. I have one, the input shaft is longer and bigger around. the bolt holes are spaced different. I could be done, but you would need to pull every thing form the flywheel to the trans to make it happen.
Check out this page for some pics of the NP 540 trans;
GMC Transmission Data , scroll down to "5-Speed Transmissions". Also be sure to check out the "Figuring RPM to MPH" info on the same page. then look at this page for pics of the bellhousing and clutch;
GMC 401 Magnum V6 Engine , the bellhousing is for a GMC V6 but you would be looking at the same differences on the back side were the trans mounts.
Also these old OD transmissions have a
Strange Shift Pattern strange pattern.
I'll be putting that
Spicer 5831-B Auxiliary Transmission in my one ton, it will give me an overdrive & Underdrive. The trick to installing one of these is to be sure to get all the parts for it from the donor truck.
