John;
Glad to see you found the problem. If I were you, I would pull the head and check the head gasket and valves. It's probably a blown gasket between 4 and 6. Also to R&R a head is much easier than swapping out the motor. The vehicle is only driven 700-800 miles a year so the effort of a swap to bring about the economy of a L6 is kind of a hard to support. Of course if you are looking for a cool project, go for it! With the low mileage, how is the cooling system? Maybe corrosive conditions contributed to the gasket failure.
Good luck.
SimS
