FWIW...I used to run a six pack hood on my cuda. It ran about a tenth quicker and 1 mph faster sealed than it did unsealed. I switched to a 5" cowl hood, unsealed, and it ran the same as the sealed six pack hood. This was on a combo that I'd run for years. I have experimented w/ sealing the cowl hood to the carb but haven't seen anything really. 
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 4 speed, Dana 60
'71 Demon 340, no drivetrain, on blocks behind the barn
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
| 
