Personally, I don't hold stock in the idea that a computer deserves any more respect than any other tool. Put a rock through it for all I care. (Or don't and donate it to charity - it would be a shame to destroy a tool just because you can when another person might get some use from it.)
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. 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! ;-)
Peace,
Drew