You're really going to want to convert it to ATX. It's so easy even I could do it. I couldn't get the soft power to work, though.

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Did you use an inverter on the soft-power-on wire? Of the ATX and old style Mac power supplies, one of them uses 5V as the power-on signal and the other uses GND as the power-on signal. I can't remember which is which, but you need an inverter on that wire to make it work, if you switch from one to the other.