8 3/4 swap questions...

The 8-3/4 will get wider with the LBP axles and brakes I just can't recall how much. If the rims are close now then they will not fit. Off the top of my head an 8" wide rim in the rear will want at least 5.75" or more of backspace. You'll need to measure. takeoff 94-04 Mustang rims seem to work well here. What do you have up front as the 73-up make the front wider too.

You will need to address the driveshaft.