you will have rubbing issues if you run 255's in the front. 87-90 mustangs have smaller wheel wells as 91-93's. Also 275's will not fit in the rear unless you roll the fenders and delete your quad shocks. Biggest tire you can run in the rear without modification is 255/40/17 and biggest in the front is 245/45/17. Ask me how I know. I have 03 cobras on my fox and ive been through this
255's should fit with no issues. With 2" lowering springs, factory '84 spindles/calipers, I was able to fit 17x9 Cobra R's with 255/45R17's on them. I ran 275/40R17's in the rear, with a rolled fender lip.
I'm currently running 275's in the front and 285's in the rear on my '84 Capri RS.