KC, If it were mine, I would leave the breather on the rocker cover as long as it has a filter or screen in it to filter the air entering the system. Without it all your doing is creating a vacuum in the case.
The BOSS at the back of the Holley with the plug in is sometimes called the Gulp Valve port, it was used on cars with an Air Injector Reactor emissions control system, also known as "smog pump". They were mandatory for California cars starting back sometime in the mid Sixties. Can't quite tell but it doesn't look like yours has that. If you take the slotted head plug out you should be able to install a 1/8"NPT nipple in there and connect your brake booster with a 3/8" hose. If the plug has straight threads you may have to tap pipe threads in or use a straight thread nipple with a shoulder to get a tight fit. Then as you suggest, connect the PCV to the front port and re-adjust the idle mixture and speed screws.
Just going by my rather alarmingly fading memory, I think it's a 1/8 NPT but I've been wrong before. Maybe take the plug to the parts store when you buy the nipple just to insure you get the correct size.
So you have your vacuum advance hooked up to the port in the front metering block?
