Drive shaft swap

let's say your car had a 7-1/4 rear in it to start...with a 904 or 833 it would have had a driveshaft 51.43 inches long. now you're running an 8-3/4 rear, you'll a need a driveshaft that is 48.60. NOW, this is according to my gathered information over the years. if you measure the overall length of both your transmissions, they should be the same. the only real change was the rear end.