most of the 70-1 cars with automatics had 904s, and so did the 75s, but the gear spacing was different - wider spacing of gears in the later cars. Later cars had lower 1st gears, somewhat lower second gears and the same 1:1 third gear. This was done to accommodate relatively high (numerically low) rear end gears (think 2.45:1). The lower first and second gears would cover up the high rear gears getting up to speed but result in lower engine rpms in third gear. You should figure out which gear set the 75 has before choosing rear gears (depending on your aims).
...and yeah the doors interchange between those years but they are completely different internally