That said, you would have trouble putting a 5500 board in a 5x0 for two reasons: Machines up to the 575 (inclusive) had a shorter logic board so it won't fit physically. The 5500 requires 3.3V which isn't supplied by the 5x0 power supply (to the best of my knowledge). So even if you wanted to put a 5500 board in a 580 where it might physically fit, it wouldn't run. However, you could add a 3.3V regulator to a 580 and probably be fine.

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I try not to make this stuff up. Warning, not for those who think the 5xx or 5xxx series is sacred. -grin-

http://www.macmod.com/content/view/34/192/

But you weren't far off on the 3.3V regulator:

http://68kmla.org/forums/archive/topic.asp%3FTOPIC_ID=5097.html

And while you are at it, why not swap in the Trinitron CRT from an abandoned LC 520? Then it would be a great machine!

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This will probably only compound my sacrilege, but I would go for a transplant of the 580 guts wholesale into a useless 520 case (can we all agree the 520 was useless?). Then swap with the PPC logicboard and that would give the the best possible model/features. The reverse would be true as well, if only Apple had made one model of the 5xxx series with the Trinitron, the way it had been designed. The Trinitron might fit in a 5500 case, but there would definitely be some gaps around the sides. Though a little putty sanding and painting would clean that right up. Black really helps minimize all these behemoths.