You could also google the diff stats on the dif trannies and check the lengths, if they are diff, then the driveshaft has to be shortened or lengthened that much, same for rear end changes, if you change a early 8" 10 bolt for the stronger later one you have to shorten the driveshaft 1", you could also look at the yolk going into the tranny and see that there's play there, if it's too tight it will work for a while and then the car will start pogoing down the street because the driveshaft is bent