Uhm, if you are going 'Road Racing' you'll want to ditch the J-Bolt traction bars. They will have an adverse effect on handling, especially with the soft spring rates you are using.
Your rear spring rate is/should be determined from and with your front spring rate in mind. But with a tire of that size and the ride height issues you have, the required spring rate is going to be more than what you can get from an off the shelf spring.
As I stated, this really has to be figured with your front spring in mind.
David has considerable Road Racing experience along with lot os cone killing time and the spring rates are going to be VERY similar in either instance.
I myself run a 700ish lb front spring and run a 300ish lb rear spring. The 67 has seen a few tracks in its years and this has been the best, balance wise for serious steet driving as well as for a high speed road course. And I think David and I are on roughly the same page when it comes to these spring rate numbers and the Camaro chassis. This would be where I would start, but I dont know your chassis set-up to give you a true recommendation. I would need to know just about everything to get the weights right and balanced.
Upon knowing that, I will say that it is going to take a set of custom springs. And we (Team Camaro) have a group purchase going on at a spring shop, for the purchase of a spring that I think would be just what need.
Take a look at this thread, it talks about the purchase along with some technical info:
Leaf Spring Discussion
If you have any questions, feel free.