A 1" HVH was worth about a tenth and 1 mph on my old 446" Indy SR headed combo. That motor had a Team G manifold and a 1050 dominator on it.
Didn't gain anything on my hemi w/ the Ray Barton manifold. Haven't tried it yet on the Indy manifold.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax