I'd pick up a C5 Hardtop or C5 Z06 before a C6 Z06 if I were tracking just because of cost reasons.
The LS6 is tougher than the LS7 as well.
A bolton C5Z with decent suspension and slicks will still kill almost anything you're going to see out there save the odd GT3 and GT-R.
Miata, E36 M3, S2000, Evo are all pretty good too.
IMO you want a car with a lot of aftermarket track support and all of those have it in spades.