I honestly though no one would try because the board is exceedingly rare, and also it's a very difficult project. It took me two full month to sand down the PCB and retrace it all, and make verifications. Getting the 8 layer PCB manufactured also took a long time.
Ultimately there was the problem of sourcing the ZIF socket, and I had the idea of using a blank PCB with simple drill holes in it, and purchase some Mill Max pins that are then driven one by one in the PCB holes and surface mounted to the board.
Also, yes, the 750GX and 750GX are identical CPUs, the 750GL being the low power version. The same goes with the 750FX and 750FL, which are also drop in compatible with the 750GX. Note that the 750GL only comes in 800 and 933MHz versions, so they have always been overclocked on the PowerLogix boards. I have added some hardware DIP switches to set the PLL1, but this is probably not very useful as the PLL2 is driven by software. The PowerLogix software does allow to change the clock multiplier on the fly, and after the extension is loaded during boot.
We obviously have the same source, REI is a common source for legacy items that need to be supplied long term, often for military application. They use original masks from the original vendors to manufacture second source parts under licence when the original vendor decide to make the part obsolete. So while these look weird, they are new. The BGA balls are also shallower than the original IBM parts, I expect because they are RoHS solder balls, and the IBM ones are the traditional high melt 90Pb/10Sn balls.
Now the bad news is that the VRM of my PowerMac G3 beige did release the magic smoke when I tried it! I have traced and triple checked everything and could not find anything wrong, but it is possible that when inserted in the socket, there could be a short between either VCCcore, VCCio, or GND. Need to check if it is the high side or low side MOSFET on the VRM that blew up to give me a clue, and hopefully have a simple fix for the PCB (drilling a via on the offending connection).
Well maybe the morale of the story is that I should spend more time here!!! I hope I did not ruin any hopes or expectations, I'll gladly share what I have learned so far, and hopefully will try to resume the project as I had some major deadline to beat lately.