Welcome to pinside. I'd highly recommend you join the forums over at pinballcontrollers.com. There are lots of guys doing what you want to do, and the P-ROC is a very well established product with many success stories. A great community of guys who can/will welcome and help you through your project. I lurked over there for a year before I bought my P-ROC stuff, and before and after I did, that group was top notch.

As far as Gottlieb System B there are a number of ways to go. P-ROC runs on the same voltage as your MPU, so you can use your power board. P-ROC replaces the game's MPU, so you won't need that. The computer replaces your game's sound board, so you won't use that. P-ROC doesn't drive coils, so you will need a driver board. Maybe you can use the driver board with a bit of hacking, maybe you can buy replacements, etc. But as I say, best to head over and introduce yourself!

Best of Luck,
Dan