That valve is to allow extra fuel during high load operations. With a 6.5 valve, that extra fuel will come in when the manifold vacuum drops below 6.5 inches, if you change it to a 4.5 then it will open at that vacuum level, so it will release fule later. If you have low vacuum at idle and cruise then that may not be enough to keep the valve closed, but the result is usually fouled plugs. Put a vacuum gauge on your engine, check the reading while just cruising at the rpm you mentioned, if it's above 6.5 the valve will be closed anyhow so you wouldn't gain any economy by going to a 4.5. What kind of cam and what vacuum do you have at idle?