Welcome to the forums.
The white smoke at start up is coolant in the combustion chamber.
I've been here now for a little while and I've discovered that theses trucks can't tolerate any over heating.
You're in the 4 litre V6 forum, so I'm going to assume that is what your engine is.
What year is your truck ?
Pull the plugs, one will probably be very clean from water getting into that cylinder.
At any rate head gaskets just don't fail on there own, they fail because the head is warped and or the head is cracked allowing water into the combustion chamber.
Time for some new heads.
It's still over heating because you're loosing coolant through the combustion chamber.
The oil leaking onto the plug tells you that your valve guides/seals are worn, probably time for some new heads anyway.
Also, when adding coolant, remove the rad cap and make sure that it's full up to the neck, because you have a leak, the system won't suck fluid from the over-flow container.
At this point, most likely not the water pump, if it was failing, you would see water coming out of the "weep" hole.
You can try running it with out the thermostat, but in the end that's not going to stop your coolant loss problem _ keep on eye on the level by removing the rad cap when it's cold.
On a cold engine it should be right full up to the neck.
A leak in the muffler will not cause over heating, but that should be dealt with depending upon where the leak is and its relation to the O2 censors.
Last edited by Jeff R 1; 10-10-2016 at 11:24 AM.