How to determine a safe red-line?

The theres 3 things I'd say for you to look at for red line and bottom end is first the weakest link there would decide the limit, everything you have should be good for 6500 rpm. Second valve springs and valve float would be the limit there. And third the power band no point spinning it to 9 grand when it's done by 5500.

shouldn't really need to push your engine past 6000 rpm make sure your valve float is at 6000 or more so you get even last useful hp out of your engine.