If you own a Cyberstorm or Blizzard board you should be able to
overclock the 68040 as much as it can stand. On some older accelerator
boards you may get trouble with the board itself if trying to
overclock.
I currently run my 68040/25 at 40MHz and it has been running fine for
several days now, and it doesn't generate much heat. I've added a fan
to the CPU before I put the lid on my A3000D.
I have been thinking to try 45 or 50MHz. Have to get some oscillators
to do that though. (I love the smell of burning sillicon)
