In your case with a stock suspension you should be perfectly fine out back with the more common 6.5" BS 17x10.5 or 18x10 wheels so long as you keep fox length axles.
18x9 Saleens will fit perfectly front and rear with out any modifications. Though you may need to run a 245 tire up front in order to keep the tire from rubbing the strut. A 275 tire on a 9" wheel depending on your alignment may require the need for a 1/2" wheel spacer. I have seen some wheels clear the strut with out a spacer while on others rub the plastic top dust cover on the strut tube,.. and have a tendency to rub the fender liner and lower control arm during tight turns. If your car is lowered then I would definitely stay with a 245 tire up front. The 87 - 90 cars have a smaller front fender wheel opening,.. where the 91 - 93 cars have a larger wheel opening in the front. The 91 - 93 5.0 cars came with 16" Pony wheels and needed the clearance. The 87 - 90 cars had the 15x7 wheels with only 225 profile tires stock and didnt need the clearance.