? on 7 1/4 to 8 3/4 rear end

You will definetely need to shorten the drive shaft and the best way to come up with a length is to measure the actual distance especially since you're replacing the rear. Have all the car weight on all 4 tires, put the tranny slip joint all the way forward into the tranny until it stops (gentley) then withdraw it on the splines 1". Now measure from the tranny yoke cup area center line to the rear end yoke cup centerline and give that dimension to a reputable drive line shop. Make sure they balance the drive shaft after ends have been welded.