I'd skip the rear disc brakes. A disc/drum setup is more than adequate, simple and easy to find. Rear discs are great for looks and ease of service but unless you're properly proportioned to the rest of the system, they are dead weight.
The reason new cars have 4 wheel discs is partly weight savings and for the improved ability to integrate a functional anti-lock system.
A light car like these Valiants will see no benefit from rear discs unless it is a road race inspired build.
