I made the switch once without shortening the drive shaft. If you have leaf springs, put the car on jack stands and let the springs and rear hang loose. If you can get the drive shaft in with a little room to spare, you should be good to go. If you come up with not enough room, you can probably find a way to move the rear axle back a touch with a spacer between the body and front spring hangers, or on a ladder/fourlink just screw out the rod ends IF you have plenty of engagement left.
Be sure your motor tranny combo can't move rearward under load if you are going to cut it close.