I am also going with a manual master cylinder.
I have never had an A-body with power brakes before and if I can stop an A-body with manual brakes then anyone can.
I am not sure how my manifold would work with vacuum accessories would work anyways....I have a Dutra Hyper-Pak manifold for this car so I also need the space so I doubt power brakes would work anyways, but mainly I feel I don't need them