340 HP's vs headers

As I understand it large tube headers can hurt bottom end torque on a milder "STReet" ENgine by reducing back pressure too much.A friend of mine years ago had put the largest diameter headers he could find on his 273/235 NHRA stock automatic car and realized minimal gains, He swapped them for a set of Stahl 1 1/2 tube headers and picked up 2/10, Down load camquest6 at compcams.com and and plug in your motors values and then see what the difference would be between large tube and small headers.