The 340 is out....again.....
Good luck on round 3. If it were mine they would have to show me in absolute certainty the problem or it would not be put back together and further more I would find someone who can find the problem. The process is no more than logical thinking.....
However if they say it could be the heads I would question them however life is about learning something new and quite frankly your cylinder heads being the problem would be something I have never seen...
JW