C5 front brakes will fit inside all the factory 17" wheels found on the 4th gens. I believe you can get C6 calipers on there as well. They are slightly bigger than C5 calipers.
One thing to pay attention to when decided 17" or 18" is tire selection. I can't see it becoming a problem any time soon that tire selection would be limited on a 17" rim but it's something to keep in mind. Other than that, it's mainly a looks thing.
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Trey
Current rides: 2000 BMW 540i/6 and 86 C10.
Former ride: 1979 Trans Am WS6: LT1/T56, Kore 3 C5/6 brakes, BMW 18in rims
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