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 Plymouth Satellite, 383, stock 906's, 3550lbs, 11.18 @ 123, 1.51 60'
WD- Comp Cams / Indy Cylinder Heads