I have a 68040 who can run at 40Mhz and a Milan with a 68040 clocked to 25Mhz.
I'm thinking to swap the original cpu by the 40Mhz but i've read the following warning from this link: http://www.der-ingo.de/en/milanhelp/hardware.html
If you want to run you Milan with more than 25 MHz properly, you'll need one very special chip, which is very hard to find. It is the chip 74ACT157 SO16, small version
I've searched what this IC do (found this link http://www.fairchildsemi.com/ds/74/74ACT157.pdf) but i already don't understand why we need this to change the clock of a cpu.
If someone can help me and sorry for my bad english.