360 header gains?

A BIG factor in sizing the tubing is the rpm of the power band.
and the rest of the combo makes a difference too
If one is setup for 1/4 mile racing like oldmanmopar just illustrated, then the rpm through the lights is going to be at or near the end of the rpm of max power.
If one has a car set up with street gearing, then through the lights is often far below that rpm.