Hey Greg-
I alternate between 5 kg/mm and 6 kg/mm on my late '74 260 w/ SBC, which is used mainly for track events....probably on the low end for a track car, but most of the venues in my area are pretty smooth w/o much elevation change. I also like to drive it on the streets occasionally. I'm using Bilsteins in the front and Tokico adjustables in the rear -- they haven't blown up yet, but I'll probably switch to Koni yellows when they do. The car weighs roughly 2780 lbs, and is balanced pretty close to 50/50.
IMHO, 200 - 250 lb/in should be a good starting point for you if most of your driving is on the streets. Road conditions and your personal preferences in ride comfort vs handling will be major factors in what setup works best for you. Good luck with it.
r/John
P.S. Forgot to mention that your Illuminas should be fine with 250 lb springs, as long as the shocks are still in good condition.