Not to totally bury you with information, but to minimize your spring collection read through this thread: Pirate4x4.com Spring Tech

There is a mountain of info in there that is offraod based, but I'm sure some of the theories and math applies to coil-overs independent of the application.

You should be able to use the current springs or the 250's and back calculate your corner weights. I've read that whole thread and the general idea is to use the lightest spring possible to support the rig with a certain amount of preload and let the shocks control stuff. I see no reason why that isnt a pretty good idea on the track as well. Offroad uses much longer travel shock, but something like 3/4-1in of preload should be a solid starting point on a 5in travel shock.

Edit: Also, working from the springs you have now should give a better idea of what rate you need than all this torsion bar / wheel rate stuff.
Disclaimer: My car has torsion bars and has made 14 passes in 4 years