Driveshaft length

If you measure from the end of the tranny output shaft to the center line of the u-joint cap on the rear end with the car's weight supported by the axle and give that number to the drive shaft shop they can figure out what you need.

Going from a 7.25 to an 8.75 requires a drive shaft that is shorter by about 2" and going from a 904 to a 727 requires another 3" or so shorter. When I did my swapping I just told the drive shaft shop to shorten the drive shaft by x inches.