Installing a boost gauge is easy on a 900. Just tap into the vacuum line that goes to the bypass valve. Any decent auto parts store should have a plastic "T" fitting. When I installed mine, I got a barbed adapter for the gauge at Home Depot that let me run a standard rubber vacuum line into the car instead of the ultra stiff nylon line that boost gauges typically come with.
Smart Performance has good installation instructions for their MBC and MBC+A products. The instructions are on the left hand side of their page here.