I have two grids for old muscle cars...
One is inspired by the classic TransAm series from 1966-72, and has only Mustangs(Boss. Mach-1 and the Shelby GT350),Camaros,Challengers and Pontiac Fire Birds in it (the Firebirds in GT7 are from 76, but the first Firebirds showed up in 1972 or so in the original TransAm series, so for me that's accurate to have them there), some with original TransAm liveries(and driver names), one Petty Nascar livery and the reast are originals by myself...
In the other one I substituted the Firebirds for Pontiac GTOs and also adedd two Superbees to the mix...
I usually use RH tires for all car and tune them around 620pp or 650pp...if I have more than one model from the same car, I tune them differently(so there are some that are light with less power, some are more powerfull but also heavier, some have spoilers some don't, some are engine swapped)...but all cars have racing suspension, racing breaks and racing break pads, racing transmission and a racing clutch...
In most of the races the Bosses and Mach-1 are dominant, in the TransAm grid the Pontiac Firebirds are on par with the Mustangs...