According to NHRA tech. directors when I started racing NHRA in 1973, Farmer Dismuke and Jim Dale, and the other directors that have served as NHRA Tech directors since them all say that NHRA only uses the engine and car specs. furnished to them from the car makers
Back in the early days of SS and stock Grumpy Jenkins proved to NHRA and probably Plymouth racing division that the piston dome height for the 1963 415 HP M.W. motors was incorrect for the advertised compression ratio. stock was between + .062 and + .065 from TRW, they needed to be + .091 to have the true compression ratio Plymouth and Dodge said they had according to the math. NHRA changed their spec. after that to allow the +.091 dome height on those motors in 1965 or 1966
The only way to get a OEM legal stock piston to measure that was to use a 1964 415 HP piston and mill the top down from +.120 to +.123 to +.091
To much politics in every form of competion