I am defintely not saying that you can't make power with iron AMC OE stuff, you can.
But it is a proven fact that if you want to make lots of power there is no replacement for cubic inches. You don't see modern pro stocks running small cubic inch engines do you ? No....they run as many cubes as legally possible in their class.
Another thing is making reliable horsepower. You have to spin a small engine hard to make as much horsepower as a larger cubic inch engine.
As far as the Iron heads outflowing the race oriented (not edelbrock) Indy 401-1 or Brewer heads....they are not even close. The 401-1 Indy heads flow right at 400 cfm and the Brewer heads even more than 400cfm.
That much head flow needs more that 400 cubic inches to take best advantage of it.
A fully worked AMC iron head can be made to flow around 300 cfm or so but that is with a HUGE amount of work that is not cost effective compared with the Indy 401 SR (stock replacement) head. The 401SR (stock replacement) head can be made to flow around 350 cfm with a max port job. But they will flow 300 or so cfm with just a moderate cleanup of the ports. By the time you payed for a professional to max out the Iron head you would have more money in it than a set of Indy SR heads cost.
If you want an AMC Iron head to flow more than the 300 cfm ballpark the only way to do it is with the cut them up...section out multiple castings and weld the pieces together tricks like Wally Booth did. Again not cost effective with modern Aluminum race oriented castings on the shelf for sale.