When I rebuilt my engine I considered going with alloy heads to replace my iron 1964 OEM heads but I wanted to use my 2X4 high rise intake. I found only a couple of heads that MIGHT work with my intake, but there seemed to be some uncertainty as to port matching.
The high riser heads have really TALL (as in long) ports compared to the MR and thus require a specific intake gasket as well. There are no heat crossover ports to warm the intake on the high riser configuration. But that may be a non issue as it seems most of todays aftermarket alloy heads don't have an exhaust cross over port configuration anyway.
In the end I decided to stay with the iron 1964 date coded heads regardless of what was available. I just felt it was a good combo for my build, date coded heads and date coded intake (1964). Some say the MR's flow 'almost as much' anyway as are 'almost as good' as the HR's. I suspect that holds true under the 6500 rpm or so. The high risers continue to flow well beyond the 7000 range. My original engine did run in the 7000 plus range and pulled strong at that lofty rpm, but was to radical for the 'street' due to BIG cam profile, race only carbs and 12.5 to 1 compression.
I rebuilt using a milder cam profile (6400 or so), smaller carbs and custom pistons that provide some 'quench' as well as lower my C.R. (9.8 to 1). I doubt that I am taking advantage of the higher flow rates available with the high risers at this point. I kept them because there 'cool' and I like the idea I'm running "Tall and Terrible" high risers with a 2X4. Very street friendly now, power comes on smoother than the 'explosive' power I had before. It's about a 1/10th of a second slower in the quarter (11.90 vs 11.99) but still 'fast enough' to take your breath away. MPG almost doubled!
I think NASCAR outlawed the heads for the 1965 racing season, Ford dropped them and they were replaced with MR's.