For comparison, when I was testing my 383....... Unported 906's, rpm intake, solid cam....... Going from 1-3/4 x 3 headers up to 1-7/8 x 3 headers picked up power pretty much everywhere(within the tested range).
More tq and more hp.

I don't recall the amount off the top of my head..... Around 15hp iirc...... On a motor smaller than what you're building, and with much lesser flowing heads.

The info is in the tech archives if you're curious.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads