Food for thought, I made a header for my 250, front port turned and ran down towards the frame getting larger as it went. Each additional port was added to the pipe with smooth transitions very much like you would see on a " dry lakes roadster " . It went from 1 5/8" pipe leaving the head to 2 1/4" down by the transmission. The exhaust was 2 1/4" to the rear bumper with a high flow turbo type muffler. The truck ran no different at all, just louder, I even went to the local drag strip when it was still around and made three passes back to back. The next week I went back with the factory exhaust system, 2" pipe, cast manifold, and stock muffler, the times slips showed no difference since the weather was still the same. The header and larger pipe went to scrap pile.
Headers only work if you run more air through the engine than the current exhaust can handle. Higher compression, way more air flow through the head, and higher rpm. The carburetor and intake IS NOT the choke point, the head is. Since I ran my engine at the exact same rpm, the exhaust didn't add any power.
My engine runs better with dual carburetors only because the cylinders are better balanced, meaning 1 and 6 are getting the same fuel mixture as 3 and 4. It has nothing to to do with more air flow or more fuel.
When I installed my stock Monojet manifold and carburetor last summer for testing, I could not get a decent A/F ratio, no matter how I jetted the carb. It ran Ok, but always showed lean, this was a distribution issue.
