Those are strange sizes to me. Well, your SN95 Cobra R that're frequently run on foxbodies is a 17x9 with a 5.95" BS. It barely clears in the front, and rubs on lock-to-lock turns. A 17x7 narrows the wheel 1" on each side, and then the 4.25 BS pushes the wheel out 2.25". Therefore, I would say that there will likely be even more clearance issues on turns. So, I do not think they will work, especially on the pre-91 cars, because they had less room in the fenderwell.
The 15x10 would probably work with some pounding, but certainly depends on the tire used (Something 26" tall in the 275 range would be your best bet. I'm running an 18x10 out back on the '93. The previous owner, Groverdill, said they were 6.25"-ish or very close to SN95 BS. So, I'd say they'll probably work. If anything, you'll be able to space them out and make them work.
Ok I think I’m going a different route. Will 17x10.5 wheels with 315/35 fit? I see people say just have to roll the fender, remove quad, and hammer the inner fender some? It that really all?