Dont do it!
Whilst pin compatible, AFAIK you cant just bang an 060 into an 040 socket since it uses a lower core voltage (3.3v as opposed to 5v), so there's a real risk you could burn it out :-o
I'm not so sure overclocking your PPC to 300 MHz is such a good idea either, but 266MHz (which I seem to recall is what the 240MHz version is rated up to) and maybe 275MHz are not so bad.
I think I'll leave my 240MHz as it is and see if yours takes 300+ first ;-)
