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

Engine Dyno's are a valuable tool, but in the end all engines have to be tuned once in the car anyways. Dyno numbers can be all over the place and vary substantially. To me, I like the E.T. slip.
How much experience do you have working with a Dyno ?
Like correction factor, what it's used for, why you need to know what it is and how it affects performance at the track.