Hi Don,
Another alternative is to "push" the wheel centers. Although my car is not a GP (except for the body)I still had outrigger tires on the front -- even though I had 4.5" wheels on standard ball joint torsion beams.
To fix it, I had the steel wheel centers cut out of the wheel "hoop" and repositioned with 3/4" less offset.
Since I built a custom IRS rear suspension, I didn't have the problem in the rear -- I was able to build the rear chassis to fit the body.
Anyway, I have some pictures on this site (click on "Photos" above). Look for rs-60 mark, about 7 lines down. I have 4.5" rims on the front ("pushed") with 185/15 tires, and 5.5" rims on the back (stock Mangles offset) with 205/15 tires.
Mark