rear engined suspension tuning (spring rates etc)

[Richie]

IMHO you have to take a step further back before investing in springs and dampers. At first you'll have to make your car as rigid as possible by welding in a full cage and/or welding all seams that are normally spotwelded.
Then you'll weigh all parts that are responsible for unsprung weight as discs, calipers with pads, hubs, wheels, tyres, uprights, trailing arms, etc. even the wheelnuts. Offcourse you'll have to know the weight of the rest off the car as well.
With that knowledge you can choose the springs and dampers. I prefer stiff dampers, slightly softer springs and rather heavy stabilizers.
Ideally you should have your car on scales and adjust the cars weight evenly over the 4 wheels, not only front/rear but diagonally as well.
Good luck!

Cheers,

Richard