At the time they didn't have a stronger axle, heck they didn't even officially have an IRS differential. That came out in 1964, except on the Gran Sport. They only made 5 of the Gran Sports and only 4 of them ever got driven / raced before the whole project was shut down. Something about an agreement among the automotive factories to not directly sponsor racing or something that happened in 1957. Crazy stuff. Somebody was making a 1963 Grand Sport replica that was factory spec for awhile, but it was beaucoup money so I never pursued it. Frame, body, and all the interior goodies to assemble your own Grand Sport Corvette. But you had to come up with a Corvette to get the drive train and suspension out of. Plus all the fiddly bits that make a Corvette run.
If you get a chance, drive a C4. Try one of the later ones with the tuned port injection. Take the time to adjust the seat so that it fits you. Don't hammer it off the line really hard, get it rolling a little first so you don't break something. If it has good tires on it, it will absolutely open your eyes. You know those freeway onramps that that go in a circle? The ones marked for 35? 65-70 easy on a dry day. Really fun to drive cars that are pretty cheap in the grand scheme of things. Go ahead Chris, wrap your ass in fiberglass, with the plastic fantastic Chevy.