It's pretty much the opposite of the way I would do it, which is wire the cab as standard JAMMA and use an arcade monitor, then interface with the PC using a JPac for players 1/2 and IPac2 for players 3/4. Then I would use a JAMMA switch to flip between the JAMMA board and the PC.
I don't think there are too many Supergun folks here and they would be your best source of info.
Also note that some JAMMA boards require -5V.