I run 400/500 on my widebody GT5 for street, I'd say start at your suggested 450/650 and go from there. It's going to be trial and error and even specific track dependent, but as Kid said shock springs are relatively cheap.
My only other suggestion is get a hold of Patrick Hals, he owns and races Candy & Andy Group 4 cars. His website with e-mail address is here http://www.detomasogr4.com/cars.html
Or call Dennis Quella at Pantera Performance Center and ask what he runs on his race Pantera (that is a tube frame car so a bit lighter).
Julian
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