your going to need an entire new sled, cpu support pci riser to convert a 5k to a 12 K machine if you want to do anything except Frankenstein it.
That's not correct. You can install an r12k pimm into an r5k machine and keep the extra disk. You just need the extenders for the cpu pimm and the little plastic jobber-do that comes with the pimm. There was something about the screws, too ... but a hardware store will fix that. You also need to butcher the pci slot plastic s0cket or it will press up against the cpu but that's not a big deal. Keep
the little MAC address chip, that's a requirement.
However, whether the project is worth doing, that's another question. I did it and then overclocked the 300 mhz chip to 350. Given the opportunity, this time I'd just look for a stock 300 mhz r5k. Occasionally you see one for $50. The r5->r10 switchover wasn't worth the trouble.
For those who are looking for the resistor placement to go from 300 to 350, I don't think it was ever published here. Old brain cells remember what they want to remember but I'm pretty sure that Dr Dave posted a little info about the clock multipliers. If you wanted to do 350 mhz, you had to figure it out yourself. It did make a little difference and if you have a 300 mhz r10, I guess the resistor switch is not that much work for some gain.
But again, having done it, at this time I wouldn't bother. An O2 as it is is one thing. A faster O2 is not that hard to come by now, not like when just the 400 mhz cpu ran $400 or more on fleabay. Usually more. For much less than $400 now you can get a 600 mhz Fuel which will kick any O2's ass from here to Manila. Overclocking an O2 is now pointless. I would suggest either a standard factory upgrade of an R5 to 300/350 mhz r5, or one of the 600 mhz edefault / Chicago Joe / mapesdhs / Dr Dave boltins, or use the thing as it is.
Or pressure one of the "I want to rewrite Irix !" fans to figure out the PROM so we can have a 900 mhz upgrade