Softer springs front will not help with traction in a rearwheel drive car.
It will make the car unstable at high speed and when braking hard at the end of a run.

As mentioned, trial and error is the only way you will find out what works and what not.
Try the tramp bars and see what happens, but I don't think it will reduce wheelspin.
Only suggestions I have is the following:
1. 225 semi-slicks 15's at rear.
2. Spin them up at the beginning of each
run to warm up.
3. Just before the run put some track bite
and a quick spin over it to increase
traction a little bit more.
4. Tyre preasure you can also play with.

See what happens. Hope the diff hold up.