That's a dangerous carbon monoxide leak, just get a new manifold, should
be under $50 at any junkyard, maybe much less if you can find a U-pull
place. Buy a set of new gaskets while you're at it, replacement is an
afternoon job requiring only basic hand tools, though a propane or mapp
gas torch is a real help getting the nuts off.

To amplify James Sweet's warning about carbon monoxide (since this is ahead
of the catalytic converter), I had a run-in with CO about 20 years ago. I
had all the symptoms except death. Believe me, it is no fun.

Mike