72 dart driveshaft

I am converting my 72 dart to a 8-3/4 rear end.The car has a 360 with a 904 transmission.I have measured it up and a 48-1/2 driveshaft would be the right size.The online chart back this measurement up as well 48.60.I have a contact that has a 68 dart gt and 340/904/8-3/4 and he has a shaft for the car but it's 49 3/8 long....I don't understand how it could be different?