My 75 Finally Gets The Attention It Deserves(This Could Take a While)

3 to 4 speed swap is going to be very easy. Should be the same bellhousing and everything bolts in the same place.

Only difference is going to be the slip yoke. The 3 speed uses the smaller 904 yoke and the 4 speed uses the bigger 727 yoke.

The 3 and 4 speed are the same length so your driveshaft will work by swapping the slip yolk.
This is assuming you have an A body 4 speed