Use 2.9v. I sell a lot of systems with the P200MMX that don't even HAVE dual voltage, so the core is at 3.3v on those, and they work fine, although at a higher temp. So I put on a better cooler. And I haven't had any fail.
In fact I put a lot of 2.2v K6-2 processors on 2.8v boards and haven't had any of those fail either. Again hotter, I use cheap socket Athlon coolers on them. You should be able to run a 200MMX or 233MMX at 2.9v, or a K6-2 400 at 400MHz/2.7v.
Here's the secret jumper settings-
On the P233MMX, 1.5x (100MHz) is interpreted at 3.5x by the processor. This also works on the earlier unlocked P200MMX at 233MHz. So you don't need an official 3.5x multiplier on the board.
The K6-2 400 (and up) interpret 2x as 6x, so setting it to 133 gives you 400MHz. You could easily do this at 2.7v, if you used a good cooler. PIII/Athlon coolers fit.
Back to you Tom...