valve head diameter vs. runner diameter

[regis101]

Greetings. Working on a 2.0 FI bus engine. The intake runner measures 34 mm OD. The port at the intake side of the head measures 33 mm. The intake valve size is 37 and the exhaust valve is 33. At what size do the valves become too large to make a difference. Maybe there is a rule-of-thumb to go by? The extent of intended porting is mainly to clean up and somewhat straighten the ports. The engine is for a 78 westy retaining the stock FI.