7 1/4 to 8 3/4 conversion

Length will depend on your transmission- a factory '64 used a ball and trunnion outut shaft, later transmissions used a slip yoke; a 727 is longer than a 904, and both are different from a manual transmission- so it depends on what parts you've already got in the car, original or swapped.
Looking in my factory manuals, the specs for cars that have 7 1/4 and 8 3/4 rears differ by about 2" with all other equipment being the same; so if you have a driveshaft that was in it and working with the 7 1/4, shortening it by about 2" would be ABOUT right... but as others have said, the right way to do it is to install the rear and then measure for the new shaft, using whatever method your driveshaft shop wants you to use.
Also be aware that with this swap, you'll need new shock plates and U bolts, since the axle tubes on the 2 rears are different diameters. You also may have to deal with different U joint sizes, depending on how the new rear is equipped.