That's really high! Your Pentium II 300 is probably still based on the old 0.35 micron core... the Klamath series. If you can lower the voltage than do so. The lower the better... less heat.
The Pentium II 300's I have the 0.25 micron core... the Deschutes series. They run at 2.0 volts. Look on the top of the chip! Mine indicates SL2YK and SL2W8. I carefully selected these chips and went on a collection spree. Anyway, I run them all overclocked at 450 @ 2.0 volts. They're all at this speed!
"It is now safe to turn off your computer"
? Quantum Physics ? Light travels faster than sound. This is why some people appear bright until you hear them speak.