Which DoctorDiff brake kit are you actually running? The Stage I and II kits are just stock 73+ Mopar disks, they have the same diameter hub register as these cars had from the factory. The Stage III and up kits actually have a smaller hub register than factory. And the current Wilwood kits for these cars have hub registers that make even the factory Mopar hub registers look small.
Most Mustang rims have a 70 to 70.5 mm hub bore. The 2.78” hub works out to about 71mm, and that’s why you see most folks that are running mustang rims have modified the rims hub bore to clear. Most don’t take much, some people do it with a dremel.
Lots of aftermarket rims use a 73.1 mm hub bore, but you have to be careful still because a lot of them taper to a smaller opening for the wheel cap. So not all of them will work without modifications. The Enkei RPF1’s I have are like that, they start at 73.1mm but taper down faster than the hub on my Stage III brakes. So I had them machined to fit.
17x4.5” is kind of a specialty size. There’s a decent chance you’ll need to have the rims’ hub bores modified to fit. Even if you find a wheel with a hub bore that starts at 73.1 the rim will have a thin mounting area, so if it tapers to a small wheel cap opening they still may not clear.
Your other option would be to turn down the hub register on the hub. If you've got a Stage I or II kit turning down the steel a bit isn't a big deal, the PirateJack kits do this on their SBP kits that use the 73+ rotors. If you have a Stage III or higher kit you can still do that, but I wouldn't turn much off of the aluminum hubs those kits use.
