[swamp rat]

A crack in the block or heads or manafold is possible, but i'm pretty convinced that if cyl's 2 and 6 had oil and water in them and that you previously removed the head for a "miss diagnised" problem then the problem IS a failed head gasket and intake manafold gasket or both, and also a possibility the head bolts didn't seal adding to the problem..

Did you re torque the head bolts after you put the head back on and it was run for a bit? did you recheck the intake manafold bolts again after it was run?? Did you put thread sealant on the intake manafold bolts that pass into the heads water jackets? if not tho's issues could cause a major leak which would fill the cylindars and crank case and also explane the vacume bouncing allover the place.

As for why one valve failed its hard to say, if the timing was off i would think more of the valves would have contacted the pistons, or more pushrods would be bent so i'd rule that out. Is it possible you didn't have the pushrod properly adjusted or didn't have it properly in the lifter cup and rocker when you reinstalled it? its the only thing i can come up with along with the possibility bad lifter but a bad lifter wouldn't cause a broken valve.

Yes water flows into the intake on a 350 (assuming its what you have)

Now because you imbedded the valve head into the piston that piston is shot and needs to be replaced, that cannot be accomplished properly without a full rebuild with proper machining of the crank and block and now the heads by a machine shop followed by all new pistons, rings, valve job ect....

To be honest i think you should take the thing to a shop and let them rebuild it for you or buy a allready rebuilt engine because your "miss diagnised problem" and subsequent repair work is probably the root cause for the failure.. probably why there is a lack of response by other members here in 4 days. Good luck!

EDIT: I just wanted to add that i'm not trying to be harsh with you or discourage you in any way cause we all have to learn sometime, but i think if you want to proceed with a rebuild you need some guideance from an experianced mechanic and not a book because there is just too much that a book won't cover.