It might be more economical to just buy a G4 Powermac these days.

The Powerlogix 1Ghz G3 ZIF runs at 66Mhz too

http://eshop.macsales.com/item/PowerLogix/PFZGX10001M/

If you are stuck running at 66Mhz on a 1Ghz G3 @ 66Mhz why not buy a 1Ghz G4 upgrade for a PCI Mac for $96 which runs at 50Mhz and has 2MB cache onboard to upgrade a nice 6 slot 9600?

http://eshop.macsales.com/item/Sonnet%20Technology/PPCG410002M/

Personally I think the money is better spent on a real G4 these days since the prices are dropping fast.