Drive shaft length ???

Not that it helps tremendously, but I swapped my 8 1/4 out for an 8 3/4 and the length was pretty much the same between the two. I ended up getting a new shorter shaft because the one that was in it was too long to begin with, but I'm pretty sure the fit was "equally bad" between the two rear ends with everything else the same before I got the new shaft. I did it on a 67 Dart, but the theory should still hold. If only the rear is changing (engine and trans the same) then the only thing that you need to check is axle centerline to the center of the pinion U joint on both rear ends as that's the only thing that is changing in the system.