With the slot loading iMac, the boards should all physically interchange but different ones had the processor in different spots which may raise a heat sink issue. A few iMacs had a raised spot on the metal divider panel so that it would touch the processor. Most of the iMacs I swapped motherboards on had a small piece of aluminum that screwed onto the divider panel so that it touched the processor. When I was swapping motherboards, I used to move that heat sink to the appropriate spot if the processor was in a different location