Having built several K6 machines, and still have one up and on my LAN, I would strongly recommend the "FIC 503+" I've taken a K6III 350 to 450MHz with no problems at all. And a K6III+450 to 616MHz (112FSB), with just good cooling and a BIOS update. Just follow the standard overclocking procedures and you "should" see 600MHz plus.
A couple of things to keep in mind. First of all, the general consensus is that the K6's are not known to overclock very well (your mileage may vary). Second, the later, higher MHz K6 CPU's were reworked to recognize the 2X multiplier as 6X. And lastly, like all AMD chips, the K6's tend to run hot, so make sure that you have adequate cooling. A socket "A" hsf will slip right on them, I have an FOP32 on mine at the moment.