The manifold-to-head WILL leak if a manifold bolt has backed out. You can probably fix it by simply tightening the offending bolt(s). They DID come from the factory with sheet metal 'locks' that bend up over the manifold bolt heads to prevent this from happening. If the engine has been R&R'd, there is a good chance these were omitted. It is especially common on #6, where the bolt holes are farther apart than the other 5 cyls. The parts breakout in the GM computers shows the sheet metal locks, but shows all 6 being the same. This is incorrect. So, you have to go to a dealer and badger the parts guy into looking up several 4.3 applications 'till you find one that shows the correct sheet metal lock for #6.
How to find the culprit? Plug the exhaust pipe, and idle the engine. pressure will build until your leak will be blowing strongly, probably hissing or whistling as well. Look for carbon on the spark plugs. If the leak is at the manifold-to-head connection, the spark plug will have a ring of carbon on it.
HTH
