No reason to feel like you're running blind building a CR. Drop down to Manufacturers links to Classic Roadsters II, and we're all there. Don Scott is the owner of CRII, and returns e-mails.
Couple of ways to go on rear springs. CR has regular Blue Coil springs you can get. They also have a coil over setup. The original spring specified was from a Chevette. Moog makes (or at least used to make) a load leveler spring for the Chevette (PN: 663, or 633) available at any auto supply store. They're progressive wound, so cruising is gentle. Cost when I bought them was ~$70/
pr. Still in car and working fine. No modifications, take 'em out of the box and drop 'em in.