7 1/4 to 8 1/4

Im lookin to get an 8 1/4 for my duster i finally found one. it came out of a duster but a 75 i dont think that matters same length, before i get to excited about buying it. What would i need to mod. to get it under there, would i have to shortin the drive shaft? the car it came out of had a 904 in it. its been about a year an a half of not having my duster running and really excited i found a really good deal on a rear end.