I have a 05 f350 6.0L . Recently turned heat on in truck and it blew cold air I found it was low on anti freeze. I put 2gal, in to proper level. Now I have white smoke after truck warms up. The motor runs fine. Also just filled tank. Do I have bad fuel or I fear the worst blown head gasket.

White smoke is usually symptomatic of a bad gasket. Check compression, could me another leak source. Some older cars had water heated manifolds and could suck water into the engine that way.