Were the ex ports tested with the appropriate sized flow tube attached?

I really wouldn’t too be concerned with the ex flow flatlining at .550 lift........I’ve seen heads that came off good running hemi motors that do the same thing.
IMO, that’s not your problem.

I also agree with Andy on trying the single 4500 intake.

Very shortly after B1’s came out I dyno tested a 446 with a set that just got a mild bowl blend. It made just over 700hp, which seemed fantastic at the time.
In a 3000lb back half Cuda it went 9.40’s........this was in like 1991.