340 w/ 650 double pumper ?

Charlie Double pumper carbs are best for manual tranny cars. You can get them to work on auto tanny cars, but it is harder to dial in. The 650 IMO would be better, but not really worth the money spent.

If the car is not makeing use of a stock air cleaner, I would think about a different route. Try a K&N stub stack and extreme top ontop of as much filter hight you can get under the hood and keep the 600.
Betwen the stub stack helping out in add cfm flow through the carb, the extreme top also allows air to go straight down the carb.