Top End Miss

Had a nearly identical issue with my 440 powered Cuda. After a season, like yours, of changing and tuning, found that the mild cylinder head porting I done, had caused them to absolutely stall at 5500 rpm. Hot, humid weather seemed to magnify the issue and that August I knew what it had to be.
Put the heads on a flowbench at the end of that season and was relieved to find the problem, finally! I've not had a need to dig in the ports of a Slanter, but they are from about the same engineering timezone.

Good Luck!