no[little] power brakes assist

The amount of vacuum will basically not change as the engine breaks in. It's dependent on the camshaft.

The rule of thumb is you'd want about 18" of vacuum for power brakes to work correctly. You may be able to get by with having a vacuum canister if it seems to work well after you've cruised for a while.

Otherwise, I suggest just swapping to a manual setup with 15/16 master cylinder assuming you have the basic large bolt disk/10" rear drum setup.