As stated by by doublebeetx, that is a vacuum booster noot a hydroboost. To mount that vacuum booster to your firewall you need one of my booster brackets shown in this fro sale thread.
http://www.67-72chevytrucks.com/vboa...d.php?t=339081
However I do not know what the mounting stud pattern is on that booster. You will have to check that and see if it is the same as the holes in my booster brackets.