[77newell]

These engines don't have the usual head gasket we think of that covers the area between the head the the block, instead each cylinder has a couple o-ring type seals between the removable cylinder and the block. If pressure from the cylinder is getting into the coolant then probably the top seal has failed. That would require removal of the head (after you have figured out which one), pulling the cylinder, and replacing the seal.

I would consider an in-frame primarily based upon the condition of the cylinders and rings. Having said that, if you have to pull both heads you might consider doing the overhaul since pulling the heads, at least in my case, involved much of the total labor due to access difficulties. The overhaul kit was less than 2500 for my 6V92. If there is any evidence of the engine having been overheated then this failure of the seal is typical and an overhaul makes sense. If you do have the overhaul make sure they check the flatness of the heads because head warping frequently follows overheating. It is also possible that you have a cracked head.

As regards to the "problems" with this engine it may be helpful to list what you've been through and see if there might be a common cause tying them together.