I have an 89' Z and I put Focal 17"s on my car and they look great i think. My car has the same bolt pattern as a 99' or newer mustang. Don't know if all years 84-89 would be the same but if they are, any aftermarket mustang wheel should fit. Heres a look at the wheels on.
One other thing....If you need to bring your wheels out to become even with the fender wells, I recommend H&R hubcentric spacers. Check what your gap is after putting your new wheels on then go from there on the correct size to buy. Again the H&R spacers from any newer model mustang should fit.