Based on the old Commodore CP/M cartridge, I hooked the Z80 processor directly to the 8MHz dot clock of the C64.
However, at least if you don't switch off the VIC, the rest of the C64 still works with only 1MHZ, so the access to RAM, floppy drive, etc. is still limited in speed.
A big other problem was the compatibility of the old original module. I have the following boards in which I could test my Z80 cartridge:
ASSY-NO.250407 / ARTWORK-NO.251137 / REV.B
ASSY-NO.250407 / ARTWORK-NO.251137 / REV.C
ASSY-NO.250469 / PCB-NO.252311 / REV.4
ASSY-NO.250469 / PCB-NO.252311 / REV.B
SX-64 (I would have to unscrew it to check the revision)
(All PAL models)
I have changed the dependencies of the signals in some places, so that the VIC and the Z80 card should not get in each other's way. I also didn't use a counter for the waitstates, but determine in which cycle the Z80 is currently and then set the corresponding waitstates depending on the VIC.
This has increased the stability enormously. Also now the version/revision of the VIC seems to play no role. At least I couldn't find any difference in the stability of my different versions.