The 1.85 valves are enough for this engine. As the 403 is a low-Rpm engine anyway. The intake runners for the 350 and 403 are the same on the factory intakes(or close enough) that it shouldnt be an issue. Unless you plan to have very low gearing, or race it full bore past 6000Rpms, dont worry yourself. However, compression is an issue with the 403, and so is cooling. Keep a thin head gasket, and be sure to triple-core that radiator. Hope this helps.