You shouldn't have any rubbing issues as long as you run the appropriate tire size. For the front, we recommend a 255-40-17, and on the back you can do the same, or run a 275-40-17 with no rubbing issues.
Thats a fairly loaded question! The answer is - it depends. Tire size, lowering springs and alignment all play a part. A mild lowering spring (Steeda, Eibach Pro, etc) coupled with a 255/40/17 tire and proper alignment with caster/camber plates will likely not have too much of an issue.
With the smaller fender openings of the 79-90 Mustang, running the 9" wide wheel is always a bit of a challenge. You can loosen the front fender to core support brace, pull out on the fender and retighten the brace to gain some extra clearance.
You'll also want to pick up a set of rack limiters to keep the wheel from hitting the control arm on tight turns. LRS-3504HDW