Identifying Exhaust Manifolds

[harnalien]

I have a question as to the difference b/w 350 and 305 exhaust manifolds (i.e. bolt pattern). It is an '84 chevy shortbed. Before i bought the truck, the 305 was replaced w/ a 350, and it may still have the original 305 manifolds on it. i've heard that the 305 manifolds aren't an exact match to the 350, and that could be the cause of the exhaut leak i'm hearing/feeling. I thought it was bad gaskets. my question is is there any way to visually tell the difference b/w the manifolds. On the other hand, i would like to just get headers, but my truck has that opening on the front of the passenger side manifold that goes to the snorkel coming out of my air cleaner. I never see that opening on headers. is that part of the emissions equipment? if i can get headers, do i need to do anything to protect the opening on the bottom side of the snorkel that the vacuum switch is in?