4bbl

Unless you plan on enlarging exhaust system all the way to the rear bumper, upping compression, swapping stock cam for more lift, and possibly installing oversized valves, in other words building an engine that breaths better, you’re just going to see a drop in fuel economy, and not much performance increase.

A stock slant six can’t pump enough air to take full advantage of a 390 cfm carburetor, and the secondary’s will probably just begin to crack open as rpm’s hit red line.

Headers have no prevision for manifold heat, so when temperatures dip much below 40 degrees, you will suffer drivability issues with carburetor ice, and real bad fuel economy.

Best bet for a stock engine up grade would be to duplicate a proven factory super six setup; a two barrel carb, and a larger exhaust system keeping stock exhaust manifold. It’s simple, a lot less expensive, and most likely you will see a fuel economy improvement as well as better performance.