I have tried running an 060/60 chip at 80Mhz on my blizzard. At first
it seemed to be booting but then it just reset itself constantly. I
tried accesing the early startup menu and soon discovered that it was
crashing when setpatch was run. After some more experimenting I
discovered that aything that enabled the FPU would crash the system.
Prior to putting the 80Mhz crystal in I had put a modified Pentium 1
heatsink and fan on the chip and it didn't even feel warm to the
touch. After some reading up I discovered that the FPU is not really
capable of running at as high speeds as the integer unit. This is why
the 75Mhz version do not have an FPU.
I managed to run a couple of demo that didn't require the FPU and I
was quite impressed with the results. I think the WB3.1 floppy booted
the system OK so I think setpatch from OS 3.1 is okay and it may just
be a problem with later versions such as 3.9.
I think 70mhz is a safe bet although It may be worth trying 75mhz if
you can get the oscilators cheap enough, I had to pay over £10 for my
80mhz unit so I wasn't too keen to experiment further.