Yeah, is the gauge at least making it up to the center of the gauge where the little black rectangle is, which indicates maximum atmospheric pressure? If not, suspect either a bad vacuum leak, which would make the truck run extremely rough in any event, or a problem with the gauge, like the hose isn't hooked up. When I put a MP blower on mine, I was having trouble with no boost. It was due to the ring on the back of the blower drive shaft slipping on the shaft itself, so it wasn't turning the blower at all. Take the blower belt off and turn he snout pulley by hand. You should feel some resistance, since the rotors are heavy. If it spins very loosely and freely, that may be the problem.
With the snout pulley turning, but not turning the rotors, the needle would only touch the bottom of the black center mark on the gauge, would not go into it.