Only if you have 65% parasitic losses going through your drivetrain.
So, if you're running a 727, then maybe. LOL! :happy2:
The rocker figure comes from stick shift mustangs, with about 15% drivetrain loss (at that power level). So, 9-10 flywheel horsepower. On a low power car that's worth .1 at the strip on a good day.
Don't forget that the magnum heads need clearance for pushrod angle if you're running them with a flat tappet camshaft.