How to determine a safe red-line?

Cam is actually rated to 5800, I was wrong, and heads are done at around 5400-5600, so 6 would be a more conservative safe limit.

Thought the cam got trashed a few days ago because on startup I had a loud ticking noise, this was after working the motor a bit the day before, and what made me want to find a safe redline. I took the valve covers off expecting the worst, but nothing was loose, and I was getting full lift everywhere. Turns out I had a pretty bad header leak, tightened it up and no noise. Definitely gave me a mini-heart attack when I first happened!