Hi Nick - how goes it 
Just to confirm:
- iMac slot-loading 350 - 500Mhz (ceramic CPU) have interchangable logic boards
- iMac slot-loading 500Mhz - 700Mhz (IBM 750CXE G3) aka "Pangea" motherboard is also interchangable
Mixing up the two is an exercise in frustration; while the two motherboards are electrically compatible, physically they are not without some serious modding of the iMac chassis/tray. This involves removing the old raised rection on the tray where the CPU once was and relocating the heatsink. I'm not sure if you can remove the tray and put it in another iMac, but getting to it isn't easy and you'll most likely crack the plastic doing so. I don't think it's worth it.
I was given a Flower Power iMac 500 recently which is definately a keeper (pastel blue and pink Apple logos!), I had a spare iMac 600 board which I considered putting in there, but decided against it due to the extensive modding required. Got 550Mhz out of the original board though; thought it was best kept original instead of being a FrankenMac.
JB
Just to confirm:
- iMac slot-loading 350 - 500Mhz (ceramic CPU) have interchangable logic boards
- iMac slot-loading 500Mhz - 700Mhz (IBM 750CXE G3) aka "Pangea" motherboard is also interchangable
Mixing up the two is an exercise in frustration; while the two motherboards are electrically compatible, physically they are not without some serious modding of the iMac chassis/tray. This involves removing the old raised rection on the tray where the CPU once was and relocating the heatsink. I'm not sure if you can remove the tray and put it in another iMac, but getting to it isn't easy and you'll most likely crack the plastic doing so. I don't think it's worth it.
I was given a Flower Power iMac 500 recently which is definately a keeper (pastel blue and pink Apple logos!), I had a spare iMac 600 board which I considered putting in there, but decided against it due to the extensive modding required. Got 550Mhz out of the original board though; thought it was best kept original instead of being a FrankenMac.
JB