You maybe able to find out what the driveshaft length "should" be but there are no guarantees that's its going to be that dimension. The only thing that you can be sure of is that when switching from a 7 1/4 or 8 1/4 to an 8 3/4 you will have to shorten the driveshaft. Position the car with full weight on the rear wheels everything is bolted in ain't nothing going to be changed then slide the tranny spline shaft yoke all the way into the tranny then bring it back out 1/2 the distance. Now measure from the tranny yoke centerline (the center of the hole that the u joint cups fit into) to the rear end yoke centerline. This is the dimension to be given to a drive shaft specialist. If the so called specialist gives ya a funny look when say you want the drive shaft balanced find another specialist. I always knock the old balance weight off the shaft before it goes to the shop.