Rear:
American GT
Grand Turbo ZR
p285/40zr17 100w
on about a 12" wide wheel, disc face to lug mount is 1.5" to outside of wheel is about 5"
Front:
p255/45zr17 98w
11" wide wheel, 1" disc face to lug mount, and 5" to outside of wheel.
Pretty bad looking set up, IMO, but I'm assuming the fenderwell opening is different between an '87/88 tbird and a 1998 mustang GT?
I wish they would fit but i can't say Yes or No because i really don't know you got a picture of the wheels?