ADDING A 4BBL..HELP Please !!!

I am just saying a 390 and headers will run better and should get better mileage than a 1bbl and stock exhaust. When I do the calculations for a 225 CI engine at say 5000 rpm they will use effectively about 275/6 cfm so it would be over carbed by about 115 cfm which isnt enough to make that big of a deal.