8 1/4 to 8 3/4 driveshaft changes

i have a 72 duster and i originaly had a 8 1/4 rear end out of a 73 dart but it is messed up, so i put a 8 3/4 out of a e-body in it and now i started looking at the drive shaft whe i took the old rear out i just pushed the drive shaft into the tranny and it left about a 3 inch gap and now the new rear is in and the gap looks the same and i pulled the drive shaft into the new rears yoke and it went right into place. are the drive shafts for the 8 1/4 and 8 3/4 the same length. i have to take it to the drive shaft place any ways since the new rear has the bigger yoke i was just wondering if i need to have it shortend or if it will be ok how it is?