If it has completely original wiring, it is super easy to install a 60-in-1. You can just pop one of these boards from lizard lick onto your original harness and it allows you to use the start buttons as the 60-in-1 buttons.
http://www.lizardlick.com/Lizard-Lick-Deluxe-Jamma-to-Pac-Adapter_p_118.html
I'd be careful if you're trying to do a plug and play jamma set up because I found this board didn't have enough power to run my gallag board. The board has onboard rectification, but I'm not sure off the top of my head what the specs are. There's also a video here that you can check out where it explains it a bit.