Those exhaust manifolds have to be the choking point now on that engine. Dollars to donuts,headers and that little monster see’s more horsepower at higher rpm. Then a ported manifold and bigger carburetor will get even more.
I'm sure more can be had with headers, but he explained that very well, I thought. "I'm using what I have, what my customer gave me" and he did pretty darn good with it.