i've been searching on here and google endlessly but i can't seem to find this info so:

i'm trying to do a vacuum test on my 292 with the stock autolite 2100 carb. so, not seeing any other vaccum lines i t'd into the distributor advance off the carb and got a steady 4 pounds vacuum. not good. did some research and from what i've seen that is ported vacuum and not really where i want to pull vacuum for a reading. my question is if i'm not supposed to use that, where can i get manifold vacuum? it has no vacuum powered accesories and i don't see any plugs or anything in the manifold or carb.

edit: i do see a threaded plug in the back of the manifold horizontal to the engine...

it's on a 64 f100, by the way. thanks in advance