Struggling to find a wheel dimension. Slick tires for the early GTRs, which will probably not help you, were:
255/635-18
315/670-18
I would expect the wheel sizes to be identical to the F1 LM - Driving Ambition lists them as:
10.85" x 18" Front
13.00" x 18" Rear
Our representative from McLaren might be able to help reunite the wheels with the original car, or perhaps be interested in a spare set for their stock?
>8^)
ER