Hi, The backspacing is near a 1” difference from 93’ to 15’’-17” (25mm vs 50mm) on 15’-.!'m17’ Personally, it would fit using a & longer studs, verify on LMR/AM’s guide for reference. (Below).
Differences exist on the 79’-93’, 94’-98’,99’-04’, 05’-09’,10’-14’, 15’-16’.GT’s, also differ on model type(s) was a base model, GT, ECO,GT500.. You can check your GT with a tape measure from disc/drum to well edge, where rim will end up in in your well, have the seller measure rims bolt up surface to Rim edge, showing rim position.
i’d Snot rin wider than 255’s on an 8” Rim. Foxes have room without the newer, big calipers inside, both LMR & A.Muscle have useful guides on fitment, below: https://lmr.com/products/mustang-wheel-offset-guide