Doing that is actually quite simple - keep in mind that ever since the Pentium Pro, the CPU-to-chipset interface hasn't been changed AT ALL. Only the frequency has been raised twice to 100 and then 133 MHz.
Now with the Celerons still crawling at 66 MHz, all it takes to make a Celeron 700 run in a PPro mainboard is an adapter board that compensates for the differences in pinout and voltage supply, and a BIOS that is at least ignorant enough to boot the system with any P6 family processor.
Regards, Peter