what tyres are you on now?

Curious if you've tried spring rubbers/blockers. They are really big in the states especially for biasing suspension stiffness for circle track work but are also used in circuit racing (larkham on rpm was showing how they are used in Supercars) and in street car drag racing.

You basically measure your rear spring diameter, the thickness of each coil and the distance between each coil. Buy a pair of spring rubbers - https://www.summitracing.com/int/par...-212/overview/ . You can get them for standard coils and tightly wound coilover type springs. They come in different hardnesses. Install them like this - https://www.youtube.com/watch?v=CKtCpVw9F70
when you get to the track.
They basically lock out one of the coils on each of your rear springs. In FWD drag applications it increases the rear spring rate. When the rear load transfer happens at take off the higher spring rate and resulting higher rear ride height keeps the car raked which helps front end traction. When its all over, you pull them out and drive home on a standard car.