471 Big Block high rpm problems

Some more info... Perhaps this helps?
The car was running ok when I got it, all was good but:
Someone that built this motor did not torque down the camshaft sprocket, it got loose and I was pretty lucky cause all fell apart on engine startup. All exhaust valves were damaged after. Intake valves were undamaged.

After this happened I took the heads off, then it also turned out that cam was bad (2 lobes wiped) so I got new identical cam + lifters, installed new exhaust valves. Heads were done by professional shop after it happened, and were ported and put together by the Koffels place in USA, I have full documentation with flowchart for them.
I also put new timing set and new bit harder valve springs and installed 1.6:1 Crane roller rockers... The engine was running on stock stamped valvetrain on camshaft brake in. It started right up, no problems with brake in. I put Crane rockers after. I set the valve lash to zero lash + 1/2 turn.