You need to move the spring perches if you're going to do it right.
And ditto on needing new shock mounts and U-bolts, the 8 3/4 has 3" tubes and a 7.25 does not. You'll also need to have the driveshaft shortened by about 2.25", but you'll want to measure for the driveshaft length on YOUR car.
I converted my '74 Duster from a BBP 7.25 to a 68-70 B body 8 3/4. When I did it I installed a 1/2" spring offset kit, Dr. Diff sells them. Since I was welding on new perches anyway I figured I'd pick up the extra 1/2" of spring clearance.