I used the wheel adapter this morning and pulled some numbers. The outer lip of the C5 wheel will stick out 1/2" further than the outer lip of the Grand Prix wheel.
Here's my measurement rig:
The sockets are 1.500 +/-.005 tall and the measurement at static ride height is 1 5/32, so the outer lip of the Grand Prix wheel is actually just under 3/8" inside the outer lip of the stock fender.
I repeated the measurement with the Grand Prix wheel with the suspension at full droop. That measurement is 1 3/4".
The corresponding measurement with the C5 rear wheel is just under 2 1/4"... so the Corvette wheel will stick out 1/2".
Would need slight modifications actually to work:
C5 wheel in place:
Clearance to the strut:
Not quite a banana for scale:
That is the narrowest gap between the wheel and the strut. I could gain 1/8" of clearance by snagging a new pair of Koni struts and modifying them to have the sleeve seat on the original spring perch weld rather than on the clamp.
I currently have 325# x 12" springs. If tire clearance to the spring perch is a thing, I could step down to 10" springs to resolve that.
Beefy rear view for a Fiero, but no wider than stock on a C5:
Next step: grab a cheap @$$ Sumitomo 285/30 from Tire Rack and look at tire clearances.
Current tires are 245/50-16's and calc to 25.64" tall.
The 285/30-18's calc to 24.72" tall... going to lose a bit more gear than just the 3.61 to 3.94 swap. The gear swap will be 9% and I'll gain about another 4% RPM from the tire swap. But I'll also gain 4% in acceleration
The new tires will be 0.92" shorter than the old.