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That's kind of a vague question.

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.

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Depending if it's a bone stock rebuild with a "warmer" cam thrown in, you have to take in account the balancing also (might free up a few hundred more USEABLE rpm up top). If the heads
have not been "touched", 6000rpm is the norm (Magnum based motor), anymore rpm is bonus!! But it comes at a price, a motor is only as good as the builder, so beware. Parts meant for strong performance in the low and midrange portions of the tach do not produce power in the far RIGHT side of the dial! And the
REVERSE is also true (midrange - top end cams produce little to zero low end power).