The 'stand-in' 318 I put in my car came out of a 75 Truck. With the truck 2-barrel it was getting about 17mpg in the dart. On the truck it got 12. It had large venturi for a 2bbl.
When I switched it over to an LD4B and the '65 4-bbl, it went from 17 to about 22 mpg as long as I kept my foot out of the carb.
The Carter Competition Carb is probably sizeably bigger than the '65 AFB on the primary's.
If you do go with a 2bbl, it should be the smaller style 2bbl. JMO.