Taper in an inlet tract...where is best?

[RedLexus]

On my fast road Mk1 Golf 1800GTi , I'm fitting 38mm motorbike Carbs , on a custom made stainless steel manifold . The Inlet valves are 40mm , which suggests the ports should be about 36mm I think? Therefore I'll have to allow a taper in the inlet tract somewhere.... Where would be best? In the head or in the manifold runners? The car will be using a 4-2-1 Exhaust , with 35mm Ex valves , and some porting and tidying up of the valve throat area , which is predictably , poorly finished from the factory .