Exactly Correct!!
I have 200 sets of the part # Heads we have use our various Stock/SS cars and have disposed of at least another 100+ Sets.
You would be surprise how much the same # heads vary on a flow bench = Also true on the intake manifolds!!
Often there are as many a 10 cars in each of our classes - all within a .1 of a second.
In Denver @ Nationals some year back - I won a class race against a 340 Cuda
heads up by .0001 of a second true margin of victory. You could not see who won
with a naked eye! For every second we are on the Track = Ther is a Week's work in the shop!!