So….more header talk….this had me scratching my head

Actually Calvin built his headers for the engines he was given.* In fact he often mentioned that he rarely got any opportunity for input on the engine. One of his main working points was that "blowdown length" is "king"
While one doesn't want to restrict at maximum output, it is just or more important to keep velocity of the mass flow moving away from the port through the entire rpm range of interest. This is what Mr Hardy is driving at when he's talking torque.
See Novice exhaust question: What is "blowdown length"? - Page 2 - Don Terrill’s Speed-Talk

With a 4 into 1 design, pipemax will show pretty much the same. We just dont get the explanation from Larry to the extent we did from Calvin. In general their findings agree, even though Calvin did not do engine work and did much more roundy round. Larry also has a small forum for pipemax users. I havent been on for years but good stuff there.

But the simple answer is plug the numbers into pipemax and you'll see what sizes pipes are recommended for what you are doing with that engine. Then you can see if the headers you're looking at will move you in the direction you want.
___________________________________________________________
*eg 1 5/8” headers and 500hp? - Page 7 - Don Terrill’s Speed-Talk

**Larry Meaux regarding some sizing relationships Bill Jenkins , Larry Meaux and header wisdom. - Page 3 - Don Terrill’s Speed-Talk