I've seen Camaros with similar exhaust tips. Looked about 1.5 - 2 inches high. Probably could get them lower and wider. Something like this:
Also didn't the original Pontiac GTOs come with a lever so you could open the exhaust? It's probably not street legal anymore. I have seen street/strip cars with blockouts you could remove to open the exhaust system up at the track.
Those tip look good on the Harleys I've seen with those tips but if you wanted to stealth the exhaust entirely probably easier to just cut the tip back under the bumper so it's even and dumping straight down.