I'd do it when the power is off of course ...
I don't think it will ruin the board, but you may not get proper output, as the JAMMA standard is set up for mono only, and more than likely your SuperGun is set up for JAMMA standard. One of the four-pin Molex connectors on the left side of the board should provide stereo output directly - you can try to tap off of that to two different speakers (I want to say 4 ohm speakers, but I can't remember off the top of my head). Or you can wire up a headphone jack to the other 4-pin Molex and use stereo headphones.
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Cheers - Mike
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