Mine works great at 600Mhz in an EPOX MVP3G5 with the 4/12/2000 BIOS.
If you decide to go with the EPOX (GREAT board, in my opinion) just for the K6-3+, you can save some money by going with the G3 instead of the G5, since the K6-3+ won't let you use the full 2MB Cache on the motherboard anyway...
As for the board you have now, I'm not sure if it's the same model, but a friend of mine is also using a K6-3+ on a Tyan board and hasn't had a problem. He had to flash the BIOS to the most recent version in order for it to work, but it runs like a charm at 550Mhz..