The wheels are "AMERICAN TORQUE THRUST II", 15x7 with a 4.25 (Backspace,) with a Ford bolt pattern. They were very reasonable & I purchased them from Robb Rice Rennsport alloy wheels (ph. 678 300 6959) ... mounted 225x70 Michelin h rated radials.
I had to cut the front fender lip & install a spring spacer for the right front to clear on dips & under full load. The rears fit perfect & the 225 x 70 is within a 1/2 inch of the stock rolling diameter.
The fronts would work better with the stock size(205X75) or the later 84+( 215 x60) & wouldn't need the spacer but I wanted to decrease understeer so the same size front & rear are the way to go. I think the whole package of wheels & tires was less than 900$$ & the tires are excellent & ride & handle beautifully.

I am new to the Avanti world (with my ) 7/99 purch(ase) but I am very well versed in the high performance collector car world. Some other neat tricks I have tried for my Avanti that you may want to try or spread around to other folks are:

I also mounted some Gabriel High performance gas shocks , marketed under the classic series & made a vast improvement in both ride & handling again I think it was 50$ for the set of 4 talk to you soon