Consider a big stroker. 4.375 or 4.5 inch. If you port the heads you have , get better rockers and a rollercam with more duration and lift a 4.5 inch stroker will only want 6000 MAX rpm. Re gear it, and enjoy a motor that will live a long life . Molnar 4.5 stroke crank , Molnar 7.1 hbeam rods. Rings no thicker than 1-16th . Flat top pistons with valve reliefs cut for 2.25 intake 1.81 ex with enough depth to handle a 285 at .050 intake and 296 ex with .800 lift for down the road when you upgrade to a much better head.