64 Dart - 8 3/4" axle swap

You will definately have to shorten your driveshaft as the 8-3/4" diff assy is longer front to rear than the 7-1/4".
Another thing you may want to consider is make sure the pinion angle is the same prior to bolting everything up. If not, cut the old pads off and weld new ones in to give you the correct angle to prevent vibration at speed.