who would build a 5 ton rock crawler.

Isn't that about the weight of a "bobbed" deuce-and-a-half? What does a “monster truck” weigh?

I had a 413-III w/727 in my full sized motor home. The 413-III was the truck block with strengthened webbing and other “heavy duty” features. It came with a standard 4 barrel carb.

I replaced the stock intake manifold with an Offenhauser Dual Port 360 degree manifold which featured a dual plenum that separated flow of primary and secondary barrels. I topped that with a Carter 750 cfm Thermoquad competition spread bore carb.

I did this because I was living and driving at high altitudes and I needed the carb volume to compensate for the thinner air. Getting the right sized jets was important. The competition carb allowed a quick change of jets. Remove the top of the carb and the jets pushed in on “O” rings. When I was leaving on trips where lower elevations would be common, it only took a few minutes to rejet.

The manifold/carburetor swap gave me an actual 25% increase in gas mileage based on comparisons of several thousand miles. The savings on 10,00 miles of driving paid for the manifold and carb.

Back then, the comparison standard for motor homes to see how well you moved along the Interstates was Trailways buses. After this switch, I was able to pass them at will.

POINT: You have a real “kicka$$” motor. Breathing in is where you get the benefit of the displacement by getting the fuel/air mix to every one of those cubic inches. Headers are a benefit at mid to high rpm’s. They are not the way to increase performance when torque is the issue.