Take it easy there big fella........
It is called flow bench testing, and also real world experience!
Check out www.fordmuscle.com's head flow comparison......it will open your eyes. Big thing to remember is AVERAGE FLOW rules the street.....not peak flow! Despite what you might think.....even the old Edelbrock heads do VERY WELL with all the others including AFR and TW when you look at average flow!
If you are talking all out race heads.....WHICH WE AIN"T....ya, there are real wild ones, some of which are made by Edelbrock.
Now, when a machine shop that rebuilds heads tells you: not to bother redoing the old stock iron heads...........that tells me something: mainly that you can take irons out to the level that the aluminums start at.
I rest my case.