The heater outlet can be plugged with no ill effects. In fact, Datsun plugged it at the factory for cars that did not have the heater option.

To remove the bolt, first remove the cylinder head. With that off, you can get vice-grips on the bolt and turn it out.

With water in all four cylinders, you may have a warped head. This happens if the engine severely overheats. Severe overheating causes the head to warp severely, and if so the head must be replaced. A minor overheating won't damage anything. In any case when you get the head off, place a straightedge on the face of it, and see if a 0.004" feeler gauge will fit in between the head and the straightedge. If not, the head is good as is.



Changing Head Gasket