Okay the biggest key is the backspacing of your rim or offset.
I know someone running a 28x11" Et Street on a 90GT.
I personally am running a 28x10.5 Drag Radial or a 275/65/15 with a 4-5 inch backspacing sitting under my 1989 GT and they fit but...
you MUST
1st - remove the quad shocks (don't need them)
2nd - roll outer lip so tire doesn't scrub (hammers work)
3rd - convince the inner welds in with a hammer, a 2x4 and hammer should help to push the inner walls flush and away from your tires.
When you finally mount the tires check to see if they scrub, bounce on the rear end of the car trying to bottom out the car, take the tire back off and see if you have rubbing on the back of the tire, if so continue hammering until you've eliminated such.
Make sure you have smooth edges all the way around and you should have what your looking for.
If at all possible try not to rely on spacers, it pushes the rim out to run on the end of your studs, well of course this isn't good on launches. So don't get cheap on me, Safety First!!!
Most dealers sell the rim with little to no change in cost for backspacing. | 
