All chips can go a little over spec, but how much can pretty much all masks of 68060s handle with basic fan/heatsink cooling?
Is 66MHz pretty much guaranteed? (only 16MHz, I actually have an old XC marked 66MHz)
No. You are lucky if an XC 68060 works at 66MHz. Most should work at 60MHz but even that is not a guarantee. I am assuming proper cooling and year round usage in a moderate climate.
How far can you get at 3.3v?
Does 80MHz+ really require upping the voltage?
I don't know. I have overclocked a CSMK3 to 80MHz where it started to have problems after warming up. It was the latest revision 68060 and 50ns memory so I don't think those are the problem. I also clocked the SCSI and motherboard speed separately so that wasn't the problem. I think it was probably the memory controller chip or accelerator logic reaching it's max and not the 68060 voltage but I may be wrong. It's been rock solid at 75MHz by the way.
