7 1/4 to 8 3/4 swap question about driveshaft length

hi guys,

i am getting ready to swap my stock 7 1/4 rear end for an 8 3/4 rear end. I currently have a '71 dart 2-door, 727 transmission with a 7 1/4 rear end. my question is, how much if any, will i have to cut from my current driveshaft to fit the new 8 3/4 rear end? i have the driveshaft from the other car, but it was a 904 transmission to the 8 3/4 rear(so i assume it won't bolt in without fabrication). All of the parts i am using came out of a '72 dart so everything should fit....i hope. any thoughts or info is appreciated. thanks