According to Barnett who is a builder that frequents this site and has done 50+ Mustangs:
Go with 15x7 with a 4.25 backspace (back of the wheel to the mounting surface) with a 5 x 4.5 lug pattern. He recommends Specialty Wheels. At one time the shipping was $16 per wheel if ordered individually, but $50 per wheel when ordering four, so you will need to review current shipping practices.
The largest tire on the front that won't hit slightly under some conditions is a P235/60R15. If you want "The Look" go with BF Goodrich TA radials with raised white letters. The second most popular tire is Cooper, which actually gets a slightly better satisfaction rating.