booster addition to manual brakes

I'd ditch the front drums for some disks before adding a booster, disk brakes stop a lot better, front drums just plain suck. Power brakes are no safer than manual brakes, the booster just makes the pedal easier to push.
Seeing that you're from Australia means this is probably right hand drive slant 6 car, right? You won't have enough space on the firewall for a typical vacuum booster. Every Aussie slant six car I've seen with power brakes has a funky hydro-vac set up on the passenger which I would shy away from just because of the added lines and the additional slave cyl just means more potential failure points in the system. If you have power steering you can run a hydro booster if you have enough room.