1974 Dart Sport 7.25 to 8.25 swap

7.25 to 8.25 should be about a 1.6” difference, so shorten the driveshaft 1.6”. 7.25” to 8 3/4 is about 2.25”.

Best bet is to measure it yourself once the rear axle is in, using whatever measurements the shop doing the work wants to have.