First sticky in the forum is for tire fitment.
http://www.toyota-supra.info/forums/...t-q-and-a.html
I believe the stock "sawblades are 32 offset and the later model 5-spoke are 35.... I might have that backward, but the difference is minimal. stock rim size is 16" x 7".
What the 18" rim will fit depends on the width, not the diameter. I *think* you'd want around 8.5" or 9" wide, but it also depends on the desired profile.
There are more knowledgeable people here when it comes to tires, hopefully they'll chime in.
I'm merging this thread with the fitment sticky.