I am considering buying a 72 Ford F-250 2WD w/390, 4spd. It seems to run okay, but runs REAL RICH. Lopey idle, will stall if u dont feather the accelerator pedal, real black smoke out of the dual exhausts. Has headers and a 600 cfm 4 barrel holley on it now, owner says it's been rebuilt, I don't think so. Any suggestions as to which 4 barrel carb would work the best for mileage and performance? Thanks.