410 LA small block will not rev past 6000

I have no doubt, that on an engine dyno………with solid lifters in the engine, the OP’s engine wouldn’t produce a dyno sheet that had the curve nose diving at 6k.
Would the curves trajectory be heading downward? Possibly……..but not straight down.
And more importantly……….you’d never feel it lay over at 6k(if…….the lifters are in fact the culprit) in the seat of your pants.

That might even be possible with the “best of the best” HR lifters.