The C4 rotors are not that big of a problem, the bracket that holds and positions the caliper is the real problem..... You somehow have to adapt the C4 caliper to the C3 spindles/hubs....I know for my aircooled VW there are plenty adapter kits that allow using all kinds of calipers vs drum brakes , but it's different to slow down a 1800lbs car with 50HP compared to a 400HP 3300lbs C3...... "Made in China" is not really an option....LOL
I'm not aware of a good quality adapter bracket for these calipers. For the later C5 and Z06 calipers you can buy kits but some of those are very poor quality like 1/8" stamped steel junk.....
Start with the rotors: install the rotors and bolt them to the hubs (no rims, just nuts)... You may need some washers if you run out of threads..... Now put the caliper on the rotor (good brake pads, not worn)....
Now take a picture of the location of the mounting holes on the stock C3 in relation to the C4 calipers and post it here....