It's my understanding that any swap of base C5 or base C6 wheels should fit just fine under the XLR bodywork. Not sure if the 17" wheels from the C5's will clear the brake calipers, but that's a separate issue. The 18/19" combos should be fine. The wheels from the widebody C6's (GrandSport, Z06, and ZR1) will probably poke out too far in the rear. The offset on those wheels is just too low to fit the XLR, but that's going off wheel fitment calculators available online. If you like the wheels from the widebody C6's, you might be able to get away with running fronts on all four corners, but then you're getting a slightly narrower tire.
