8 3/4 ?'s

The 7 1/4" should have smaller axle tubes so the plates and bolts won't fit. If it is an 8.25 it will have 3" tubes and the plates and u bolts will work. Also note that you will have to shorten your driveshaft as the 8.75 is longer fron the ujoint to the axle centerline.

You will have to sleeve your new perches to fit the a-body springs

You could measure your other a-body 8.75 diff and be in the ball park for pinion angle. I would not just go 5 degrees, the best way is to install in car with all the weight on it, measure the angle and tack weld them (or carefully mark) then remove it and weld