The NHRA engine spec listings have always been considered the ultimate source of truth on engine configs (and they include casting nos for intakes and heads nd even the carburetor numbers), in no small part because those rules are used to referee any/all claims in the stocker racing classes.
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Yes but there specs are still what the biggest or smallest tolerance is allowed. Doesnt mean thats what a factory engine is. If your a racer and they allow 67 cc chambers and your engines has 69cc chambers you mill them to get down to there limit just as if they say you can have a .030 overbore you also do a .030 over bore so you have the biggest bore allowed. The so called factory specs were just what Ford reported as normal range ie 67-70 cc heads. NHRA just set it to the lowest or biggest specs depending on the item. Plus they required certain casting numbers for that particular year to be legal.
I wouldnt say the NHRA spec's are wrong but they are setup so you can optomize your engine to there rules.
They are called the 'blueprint specs' specifically because the numbers are the 'dead on perfect' Ford design numbers. Of course variation is allowed and certainly does occur in normal production..but those blueprint numbers are what each component's untoleranced datum values happen to be.
That is also why you see decimals in the NHRA numbers..because they are the theoretical design numbers, not averages or the range of actual numbers taken from production parts. However, normal production statistics with a Gaussian distribution* says that inspecting/measuring a large sample of actual production items will yield, when the measurements are averaged, the blueprint design value in each case.
* however, many components are toleranced with differnt values on the plus and miunu side of the 'perfect' design datum..and those tend toward a Rayleigh distribution, so the average value for all components will not equal the blueprint value..it will be skewed toward the larger of the two tolerance limits.
