Rims sizes?

I know that 17x9 Cobra R wheel will fit an 88 GT. My friend had them on one of his cars (& it was an 88 vert GT). I don't think 10's would've fit. 9" is probably the practical max.
 
You can get Cobra R wheels in 4-lug and 5-lug (I personally like the look of a 5 lugged wheel)...

Don't go any wider than 9in in the back and 8in up front so you don't run into any "major" problems...

With your smaller fenderwell (compared to the 91-93)...the 8in will likely rub a little on full turns...and the 9in in the back might hit your exhaust on turns...as an example...

Good Luck