Someone school me on header size.

i just spoke with a very reputable engine builder about headers for my 360. It is a .30 over 360 with flat tops, undetermined cam, performer rpm intake and Holley 750 dp.

The builder says if I go smaller than a 1 3/4” header I will be hurting the optimal power output. It looks like TTI makes a 1 5/8” x 1 3/4” stepped header. Would this be what he is referring to?