Buy a few more and do handpick
Maybe you'll squeeze 720Mhz at benchmarks stable.
This is with 6x83FSB or 5x100 by default?
Or search for K6-III with 256KB cache and 180nm, that will break a wall.
By the way, your spi score is incredibly slow, my PII 366Mhz laptop can do spi1m about 50sec faster.