There's plenty of quick GIIIs out there. They've been running in the sixes for years. Probably very few street legal ones because real badasses don't build race cars for the street. And they don't race for $500. That was a decent purse for a street race 40 years ago.
Good luck, I hope you find a taker. It would be fun to see. If you increase the bet by a factor of ten, I think there would be a decent chance you'd get a race.