I'd try putting it in a 40MHz Turbo040 and see if I could get 55MHz out of it.

The T040 is probably the best 68040 overclocking platform becuase you don't run into problems from overclocking the main system bus, such as the famous 'black screen on Mystic above 43MHz' issue.

Alternately, the Quadra 840AV might be a good choice, because the bus is already 40MHz. LC630 comes to mind as well, because it's the newest 68040 Mac (and even has remedial 68060 support).