Matt - You can find some useful info from this long ago post about an issue with the SM465 not fitting the earlier bell housing opening without modification.

If you want more details on comparing the two transmissions, look at:

SM420 Info from Novak

SM465 Info from Novak

I don't know for sure on the answer on changes needed to your floor board/transmission cover, but from looking at the pics in these source documents, it's a good probability that some changes will need to be made.

BTW - Since both transmissions have the same final drive output ratio (1 to 1), you aren't going to see any top end drive changes. I think the SM465 has a better 1st gear (6.55) VS the SM 420 (7.05) but otherwise you aren't going to see any big changes through the rest of the gears. There are also some considerations that may be needed concerning the output shaft on the SM465 as there were 3 different ones. Depending on which SM465 you find, you most likely will have some driveshaft modifications required. Others with more knowledge and experience with the SM465 will probably chime in here and offer further insight. wink

~ Dan
1951 Chevy 3 window 3100
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