headers vs manifolds on a 318 2bbl difference ????

And... it goes the other way too ... big horsepower on the dyno and a snail ET slip with low MPH.

That is likely because they where looking for that big high RPM peak HP number and neglected understand how much they totally destroyed the lower end to get that big peak number.... the goal is maximizing the total area under the curve and keep torque down low not to get a big high RPM HP number.