Safe voltage increase is in the +0.2v/+0.3v range. 2.4v AHX core can run with 2.7v without problem. Overclocking a K6-2 is easier than o/c the K6-III so you should hit 450 / 500 without problem (with proper cooling).
Cooling is very important. If you are using stock cooling, you can't overclock well as you have to increase voltage (produces more heat). You should at last remove the thermal pad and use thermal paste.
Before trying o/c with FSB, try to change clock multiplier. If stable, try to get the same clock speed with FSB increase (gives better performance with K6-2 line).