I would be inclined to actually do tests on it and confirm a bad head gasket.
You stated there wasn't a PCV valve installed. If this is correct, then you are going to have white residue and steam/smoking exiting the oil fill with the cap exposed. Purchase a Motocraft PCV valve ($8), then check hose from PCV valve to ensure its not blocked, then connect it to plenum. This wil help pull excess gases and moisture from the crankcase as it builds pressure. Drive the truck, get it warmed up, and then check. Look for coolant reservoir to see if it overflowing. That is also a sign of bad head gasket. So far, I'm not convinced of a head gaske problem.

If you wanted, after getting truck good and wamed, 25-30 mins, then park it, and drain oil in clean drain pan, and look for signs of coolant. You'll know if it has it. Then if you want, just refill with oil you drained or spend $15 on 5 quarts of fresh 10 or 5w-30, and Quality filter, no Fram, and then drive till May.

Just a few thoughts...