My 383 is making about 14-15" vacuum. How much do I need for the power brakes?? All I need vacuum for is the brakes, distributor advance and the TH350 tranny. If I need more, whats the easiest/cheapest way to get more?? :beer:

15 min, 18 is what you really want.

you can buy or build a vacuum storage can. if you can find a car that had dispearing head lights they had huge cans some over 2 gallons. it could then back up your already weak vacuum.
if you can make use of a external vacuum pump you could always mount one from a older 6.2 or 6.5 diesel, they are belt drive and will make about 21 inchs of vacuum.