Here's a bad sketch I made a long time ago that shows how the JPAC + AVGA replaces the game board. However note the following:
1) Pac-Man is not, in fact, a JAMMA game. At the time I made the sketch, I just picked a popular game without checking if it actually followed the JAMMA standard.
2) When you hook up the JPAC, you should disconnect the "arcade power supply". Running the power supply without a load (i.e. no game) can damage the power supply and/or start a fire.