Help with Oversteer

[Bill71]

I need some help adjusting my suspension. I have a 1971 Camaro with a full cage, LS7, T56, Speedway 9" with Speedtech Torque Arm rear. The front has 700# springs, Viking double adjust shocks, GW uppers, Hotchkis hollow bar. The rear has Ridetech single adjust coilvers. I use my car on road courses exclusively, no autocross. Tires are NT01 with 275/35-18 front and 315/30-18 rear. I have been running the car since January and have had a bit of understeer from the beginning. I have always had a bit of tire rub in the back so I changed out the 225# rear springs to 300#. i had the car on a local track last Saturday and had zero tire rub but the car was noticeably tail-happy, at one point causing so much oversteer that I put all 4 wheel in the dirt. When I changed the springs I also raised the rear about an inch at that time. I am thinking that I can lower the rear ride height and maybe add rear tire pressure and soften the shock adjustment to help with this. Am I on the right track?