You should be have enough vacuum with that cam. I have had larger duration cams than that and the power brakes have worked ok, not like stock but worked. A friend of mine is running the Edelbrock performer RPM package on his car and has about 9 to 10 inches of vacuum, his power brakes work fairly well.
As techie said you can get one of those vacuum resevoirs for pretty cheap if you need one. I would try it without any extras first and go from there. That way you didn't buy something you may not really need.