I don't recall it being that much but the easiest thing to do is take a piece of broom handle and lay it in the u-joint groove on the pinion and measure from the center of the broom handle to the center of the axle tube on both the 7.25 and 8.75. The difference is how much you will need to shorten the drive shaft. This is not a critical measurement, you need to have approximately 1" of travel left on your slip yoke in the tranny with the drive shaft installed. So 1/8" of precision is good enough.
You should be able to bring the drive shaft you have to a drive shaft shop and have it shortened, rebalanced and new u-joints insatalled for less than $100.