"safe RPM "

The parts used will not be as big an issue as the stroke. I spun a 4.15 to 7300 through the traps until I lost a ring on stock caps with studs. I did notice a crack above #4 but no cap walk. the bore was bad on my block.
I would not do the 540 unless it was a heavy car. If you cant get it done with a 4.15 or 4.25 crank you need better heads.