How hard/whats needed to change.....

If you are going to use the 7 1/4 the driveline is the same length for the 3 speed and 4 speed but you need to put a 727 yoke on the driveline for the 4 speed because the 3 speed uses a 904 yoke. If you convert to an 8 1/4 the driveline will need to be 1.6" shorter and if you put an 8 3/4 in the car you driveline will need to be 2.26" shorter.

Chuck