Safe seems to be a function of when the valves begin to float for OHV motors. When do stock valves begin to float? I don't know for sure. I've hit 5500 on a stock motor accidentially. With improved valve train, you can go higher...

The question is: Why would you want to go higher than about 5500? Stock torque has fallen off hard after 4500 or so. Unless you're pushing more boost or actually get power in that upper range, just keep that RPM limit to a sane level. Running a "stock" motor to 6k will result in a higher ET than shifting it earlier... No reason to spin it that high.