you'd most likely get about 550Mhz from a K6-2, 600 is a long shot, 700 is a never heard of in my book, only k6's to ever possibly get that were the K6-3+ series. Only later AMD processors such as the Athlon and Duron are known to get multi hundred Mhz overclocks. The K6-2 500 regularly got 550-570 stable, with a verrrry few getting 600.
I needed to up the voltage on my K6-2 450 from 2.2 to 2.4 volts just to get it to run stable at 450Mhz. I could not get it stabel at 504 at 2.6v untill it burned in for 2 years and I switched to an AMD heatsink off a retail Duron 1Ghz. Now it runs at 504, but no more. I have heard of only a few hitting 600Mhz and it usually took lots of cooling and voltage. The K6-2+ and K6-III+ did a bit better and some hit the upper 600s. I only saw a few get beyond 112x6 for 672Mhz. Some boards had the 115Mhz bus and made it to 690. Some also did 124Mhzx5.5 for 682. I only heard of a couple making it to 120Mhzx6 for 720Mhz and I never heard of 124Mhzx6 for744, but it could be possible.
Of corse a Duron CPU and motherboard is pretty cheap and would blow away and K6-2/III you could get.