Okay, try this:
Use a 2000 Z06 Corvette (C5). Max it out. Get all the best drivetrain, brake, engine, chassis, and lightweight options. Definitely get the Increase Rigidity option. Get all the tires: Yes, everything from S1s to R5s, because you'll want to have them all available for the many races you seem to want to run this car in. Don't forget the oil change.
You'll need 500 HP on some tracks, over 620 on others. More than that may be hard to drive on the Sport tires that will be required in many races. Definitely go to GT Auto and put a wing on it, then go into Settings and max the downforce front and rear.
Use this chassis setup:
SR: 9.7 / 11.8
RH: 82 / 82
SB: 3 / 6
SR: 4 / 7
Cam: 3.0 / 1.3
Toe: -1/1
Stab.: 2 / 4
Brake Balance: 3 / 5
LSD: 10 / 19 / 8
ASM: 0/0
TCS: 3
Ballast / Balance: 0 / 0
To set up your transmission, slide the Autoset to 1, then set the Final Drive ratio for each track. You want to redline in sixth gear just before your braking point on the longest straight.
This is a great car for the Supercar Festival, but you can use it in lots of others, as well, such as the American hall events, the Special Condition asphalt events, the El Capitan Enduro, and even the dreaded Tsukuba Wet race if you use R5 tires.
Remember to give it an oil change now and then, and you'll need to buy the Rigidity Refresher Plan every 700 miles or so.