A '68 383 came with 906 heads. Get them off there and find a set of '67 915 heads, these are closed chambered and will raise your compression another point. Have them ported and install 2.14/ 1.81 valves. Be sure to clearance the tops of the cylinders where the valves open. You can do this yourself by using an old head gasket as a guide. Chamfer the bore out to match the gasket but only go 1/4" deep into the cylinder to stay above where the rings travel. Stealth heads don't look stock, as soon as you look in the area of the heat crossover at the center of the intake manifold, you're busted. Go with a solid lift cam and as stated above, find an aluminum intake and paint it. A 3800 stall converter will help too.