He had an engine on dyno and exhaut manifold to play with, with customer approuval, that's it.
He said that he had to adjust A/F with the header.

Maybe header help that combo more than others. 6-pack may have more breathing capacity and perhaps magnum manifold was limiting breathing capacity of the 6-pack.
Perhaps the header help to generate more HP with Low CR and unknown cam.

At least, he show his dyno screen, does not hide A/F ratio and other data, like many other.

I like Nick and appreciate his stuff. fun to watch and entertaining.
The quality of filming/audio/editing from his media partner is well put.
I doubt they have the production budget of engine master/powernation or other. And yet Nick is as much informative in his own way.