What tires would you suggest???

275/40-17 or 255/40-17 for the front. 275 is stock for 17" 4th gens. I ran 255 for a while because that was the size the tires I wanted to run came in.

Never paid much attention to 18's so I'll let someone else chime in on that.

Stock fbody 17" wheel width is 9". If these a actual C5 wheels, aren't the backspacing of the rears off for a f-body? IIRC the C5 ran 245/45-17 up front.

275/40/18 on the rear

its 275/35/18 on the rear to maintain a 26" tire

245-45-17 and 275-35-18 are going to give you the correct diameter and will also fit the wheels correctly. I cannot advocate a 275 on the front with only a 8.5" wheel.

if u wanna go BIG....i have a set of c5 chrome 5 star 18x9.5s as well..i am running 285/40/18 nittos on the rear..BUT...i have removed the bump stops via (torch) and also used the BFH mod.once you test drive a couple times and continue to use the hammer to tweek the inner fender which can take a little time....but let me tell ya..they look bad ass and the car hooks great with NO rubbing once your done tweeking..even under high speed cornering no rubbing!!!!!!..I just ordered a set of 15s for the rear just because i need to go bigger via NOS mod

The fenders on the Fbodies were not designed for bigger than 17s. When I see anything bigger the car looks awkward to me. I think 17s look perfect on these cars. Just a visual critique.