The Stovebolt Site Forums The Shop Area Driveline SM420 to replace 3-speed in 57 3100 (driveshaft the same)

I still have the mindset that 3-speed and 4-speed transmissions are the same length and interchange as in 47-53. I need to readjust my plan. I got the SM420 cheap, but by the time that I get it rebuilt and have a driveshaft made for it, I might be better off just getting the original 3-speed rebuilt. I like the SM420 but will never actually do any heavy work with this 57. I've been searching out photos of the different driveshafts and consulting the Chevrolet Master Parts Catalog and I'm beginning to get the picture, I appreciate the help.