Take a 75-78 L28 block. Add forged flat-top pistons from the ZX. Stick with the square port N42 head from 75-76 - especially if your thinking about turbocharging later. Otherwise you could also use the N47 round port head from the 77 & 78's. Either, combined with the flat-top pisotons will give a compression ratio around 10:1. A healthy jump from the stock 8.3. Add a decent cam - 250-270° duration with .500-.520 lift. Leave stock exhaust manifold and AFM and just add K & N and a 2.5 exhaust. Will give you a real lively engine for a reasonable investment.