Please, just stop. Get an ATX PSU, plug it in, and be happy.

Also, unless I am mistaken most older Macs use a different method to trigger a startup than the Beige G3 and later models anyway.

Just stop thinking about the 7500 PSU. If you knew what you need to in order to make a 7500 PSU work then you would not have needed to ask in the first place

Listen to us for maximum happiness and minimum fried hardware.