You may or may not do this prior to changing your oil so I will mention it. Before changing your oil, if the truck is cold, you should let it run for about 5 - 10 minutes. That way the oil will drain better and any slug will come out as well.
After filling with 6qts, you should start it up and let it run for 5 minutes, check for leaks etc. After 5 mins or so shut it off let it set for 5 minutes then pull the dip stick, clean it with a rag, then put it back in pull it out and check. You should get an accurate reading of how much oil you have.
From what it sounds like if you are having a problem reading your dip stick and because of that adding more oil you could indeed have it a little over full. That can cause fouled plugs as well as seal damage. If you find it is over the max line you want to drain some out so as to get it below the max line, but not under the min line.
Don't know if that helps, but if so great...
If you continue to have a problem with the plugs I would see a dealer about it soon so your truck stays in good running order.