I would expect most any place on the intake manifold would give you the best vacuum reading. If there is already any ports in the manifold, you could "T" into it and get your reading. JMHO
Your issue is going to be finding a place to connect the gauge. If you don't have something like a PVC valve, doubt you have one, then you need to look at the intake manifold for any kind of plug. According to what I've found it appears items 2 and 5 are hoses, so you may be able to put a Tee fitting in one of them