It'll help, but not enough to be worthwhile. If you intend to just do cruise-ins and drive the car around...go for it. I'm sure it'll drive good on the street.
Just my 2 cents, not trying to talk you out of it if that's what you want to do. I feel like your car would run better w/ a smaller manifold, but not THAT small.
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
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