Is the bike legal for superstock? If yes, CSS, CSB and MW Solo would be your main classes. You can also bump up to B classes (750)
On my first race weekend, I raced C Superbike and C Superstock. I'm on an '07 GSX-R600. C Superstock was a crash fest, though. I couldn't believe it.
+1, B has generally lighter grids in the NE if that is what you are looking for. Unless you are painfully slow, you will have someone to play with in C.
I ride an 06 R6, for the light grids and better chances of getting contingency I race all the 1000 classes. A stk, a sbk, f1, and for giggles and passing practice I race c stk.