The rpm range is dictated by the cam/intake/carb/heads/exhaust, the mass of the reciprocating lower end.... and so on.......
Reliability???? What max rpm with the heavy slugs a factory 440 used???? That's the real question. Probably not much over 6k.
You didn't mention the cam other than "mild". The package as a whole is probably overrunning the cams design. It's probably doing a "nose-over" deal where it's shifting past its peak.
