Strong short block

To answer your question, 600-650, mid '70s block sonic tested or not, I'm sure that will start something, 4.040 bore, 3/4 fill block rock on thrust side, half fill on the other. Studs, line hone. 4" stroke lite weight forged, h beams, forged pistons. Full groove mains are you'd only option unless you really want to narrow stock width bearings for raised fillets... Oiling system depends on the rpm. That is where id start IF I only had factory blocks to tool around on.