Bigger Rearend in my duster

Hey guys,
I got a 1973 Plymouth Gold Duster. I'm looking to put this rear end into my duster. Its out of an older duster or dart. Although the u joint on my drive shaft measures 3 1/4", the new rear end measures 3 1/2" the spring perches are the same and the width of the rear end are the same. I've been told to buy a new drive shaft. I don't really want to shell out the money to buy one. This is my daily driver so i just want to get it back on the road. Help would be appreciated. Thanks