You have to remember the other components on the board. They may not take an overclock to 40mhz or beyond.

That Micromac upgrade isn't 90mhz. THe CPU only runs at 45mhz. There is no such thing as a 90mhz 68040. 40mhz is as high as they went. Even 45mhz is overclocked. Also, Micromac has been out of business a long time. You'll never be able to get one of those upgrades unless you find it on ebay.