I would say you don't need 1 carb, you need 2. Tunnel rams and single carbs do not work toegther very well. somthing in the 450 cfm range would work well. Alot of ppl get that bigger is better stuck in thier head and end up costing themselfs alot of headaches...
FWIW 2 vac carbs index toegther are far ezer to tune then 2 mech carbs...