The two transmissions you mention should be the same, both SM420's. If one is something different its because someone has done some swapping.

I don't know the ratios available in those rears but someplace like Randy's Ring and Pinion will give you both a listing of available ratios and a calculator to determine speed.


1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...