410 LA small block will not rev past 6000

crackedback said
Most likely the lifters giving up. Just a guess, it's going to give up around that RPM anyways.

IIRC, Hughes puts a bunch of preload on the hyd lifters. I never set stuff that will see that RPM at more than 1/4 turn, most of the time, 1/8 turn

Yep the valvetrain is starting to get pissed off starting at 5800. Fast rate cams and hydraulic lifters are an evil match.
A lazy lobe is better for revving with a hydraulic.