The issue with vacuum is more to do with how much you have at idle, and at cruise, not so much with accessories using some of the vacuum. Bigger cams affect the amount of vacuum the engine will make. Stock motors with stock cams typically make between 15" to 19" of vacuum, and brake boosters are designed to work in that range. A bigger cam with more overlap reduces the vacuum, sometimes down betwen 10 - 12", and that may not be enough to make the booster work. Some people get around this by plumbing a vacuum booster between the engine and the power booster - sort of a reserve vacuum canister.
If it were me I'd keep the manual brakes, go with 11.75" in the front and go with discs in the rear. You can always go bigger in the front too, if you're running 17"s - the aftermarket has lots of options, other than the stock 11.75 stuff. But those 11.75's were the upgrade that make it onto B body cop cars of the era.
