The NHRA knew that advertised HP was just for the insurance companies, so they came up with a more accurate chart.
The NHRA was weight divided by HP. The AHRA went even deeper - called "Formulas", they factored in cam, lifters, two or four bbl, upholstery; all kinds of arcane sub-divisions.
"Stock" was more-or-less based on what was offered in the showroom. No trips to the parts counter, no aftermarket goodies. That would have (should have) put you into the wild and wooly Modified Production class.
"Stock" more often than not was a matter of opinion.