MrMeatCleaver

I am trying to put together my 81 300D, [novice mechanic here :p ] and the vacuum system has me confused. I have a haynes manual, but it doesnt seem to have much of anything about the vacuum, and the only diagram ive found is in a performanceparts.com catalog, and while it outlines where almost everything goes, it does not tell me where the 'central plug' goes.

Here is my question. Where does the central plug go? Or . . . Can anyone point me to a realy good vacuum diagram?