The only thing I know for sure is that the 66 has an inferior valve body. Unlike the 75, the 66 has a wierd valve body where the gear selection is either 1(L),1-2-3, or 2-3. The 75 should have the gear selection be 1,2,1-2-3. So, with a 75 you can hold each gear, whereas the 66 you have to shift out of L, and then back to low to hold 2nd gear. Sounds kinda confusing, i hope i explained it well enough. They both have basically the same internals, i dont know exactly what the difference is, but I think I would go with the 75 if i were you. I have a 66 c4 with a shift kit in my mustang and it works fine except the 2-3 shift is way softer than the 1-2 shift. Supposedly its because of the primitive valve body. Ford replaced them in 67 with better ones.
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