Please read the Q605 (LC475, P475, P476) Speedup w/Apple's PPC card file before you proceed.
This hack uses an Apple resource editor called ResEdit 2.1.3. ResEdit is a very powerful tool, but if you do not know what you are doing you can do plenty of damage in a short amount of time.
This modification should work fine on the "601 Processor Upgrade" file under System 7.1.2, but it has not yet been tested. This was developed on a machine running System 7.5
In the control panels folder find the "601 Processor Upgrade" file
Copy it and put a copy away safe from harm
Launch ResEdit and open up the "real" copy of the 601 Processor Upgrade file located in the Control Panels folder
Double click on PMCM
There is one PMCM resource (-4064 and is size 39)
Double click on -4064
You will see several hexadecimal numbers which are assigned as follows:
59 5*16+9=89 gestalt for LC475@25MHz
5C 5*16+12=92 gestalt for LC575
5E 5*16+14=94 gestalt for Q605@25MHz
62 6*16+2=98 gestalt for LC630@33MHz
63 6*16+3=99 unassigned
68 6*16+8=104 gestalt for LC475 w/PPC@50MHz
69 6*16+9=105 unassigned
6A 6*16+10=106 unassigned
6B 6*16+11=107 unassigned
As far as we know, the PPC card will only work at 66Mhz when your Mac is in LC475 mode (The jumper at position J18 must be removed). So we need to add the gestalt for a LC475 running at 33Mhz. If this is not there, then the machine is not recognized, and you cannot switch out of 33Mhz 68040 mode into 66Mhz PPC mode. You should keep gestalt 89 so things will work if you ever go back to a LC475 at 25Mhz. It is not clear exactly what is happening here as just adding the gestalt for a LC475@33MHz will not cut it. You need to also add another one for the PPC card at 66Mhz. If anyone plays around and figures out exactly what is happening please drop us a line. But for now, this will work.
Select 5C and change it to 68
5A 5*16+10=90 gestalt for LC475@33MHz
Select 5E and change it to 5A
Save and close everything.
Quit ResEdit, and restart your Mac.
That's it, you should now be able to flip between a 33Mhz 68040 and a 66Mhz PPC.