I would try a couple of easy, free things first.

Obviously pull the #6 spark plug to see if it has been "steam cleaned" from coolant getting cooked (likewise, looking at the piston head with a borescope to see it if is super clean would be useful info), but then swap the #6 plug with another spark plug, say...your #4 plug to see if the misfire swaps to Cyl #4, meaning a bad spark plug.

Same for plug wiring. Disco 2's use a wasted spark so there are always 2 spark plug wires that fire at the same time for each cylinder that you can swap without impacting any spark timing. If the misfire moves then you have a bad wire.

You can swap fuel injectors from #6 to #4 to again see if the misfire moves, which would tell you that you have a bad injector.

Same for swapping the actual coils from left to right. If the misfire moves banks then you've got a bad coil.

Anyway, all of these tests are free and give you useful information no matter what you learn from them.

 

I've read everything I could find on Disco 2 misfires, and had not heard that it only has coolant running through the corners of the block. That does cause me to look at this differently. Any suggestions on how to get a visual on the corners of the block (behind the liner) and confirm that might be the issue?

 

If you search for Head Gasket failure you will get more results about how the water flows.



With the truck still warm (cooling system still under pressure) remove plug 6, insert boroscope and look around the the block to HG joint toward the inside of the motor.

If you see bubbles you got troubles.

If not, then you have a leak in the cooling system somewhere and another issue causing the misfire which could be any of the above mentioned items or just a dirty fuel system from using low budget gas, low octane gas, or both.

 

Here's what each cylinder bank looks like regarding the head gaskets: