SLANTY BLUES

Did you clean those chambers .or is that the the way it came off?

In post 35, the very first pic, of the piston, it seems you are calling #6, yet it appears to match the one that you previously identified as # 1 in post #27, and that I can clearly see in that post as being the frontmost #1 cylinder.I did notice the flecs in 27.
So I'm confused
From the headshots,It's clear to see how water got into the #6 chamber, and how it could have transferred over to the #5. But numbers 2 thru 4 are also steam cleaned,
#1 sparkplug looks normal but rich.#5 and 6 are glazed from overheating, too clean, and appear to have traces of aluminum on them. Go back to #27 and look at #2 piston, see that brown trace at the bottom around 7 o'clock ? Roll that piston to the top, push it up as far away from the cylinder wall, and check that top land. And any others that might have such a brown discoloration.
Are you running a vacuum advance?

Ok, my guess is the decks are not flat, and are too smooth. I think it all started with a bit of detonation, and the clamping force could not keep the gasket in place with the super high cylinder pressures, the pressure lifted the head and it escaped into the cooling system, where it blew some water out and this started a chain reaction.
First you saw the steam, pulled over, and shut down. This allowed water from the higher pressure rad to now escape into the cylinder(s). After it cooled down, you added water, and hit the starter, at which time the engine had to battle the water out of there. Then for as long as you had it running with water running out of the tailpipes, steam-cleaning was going on.

This is my OPINION, I could be wrong.
But if I am right,
there are at least two pistons that have lost some aluminum.
there may be 5 bent rods
you need to check the deck and the head for flatness, in every direction, and probably re-machine both a little rougher.
You need to figure out your exact Static compression ratio at the time of the failure, and figure out a way to reduce the Dcr/cylinder pressure, so this doesn't happen again.
Then you need to fix your cooling system. You have a 30* rise from minimum running temp of 160, to a running temp of 190, before pulling a load; your system was in compromise before you ever left home.
Then you need to feed your carb fresh cold air. By it's being in that hot doghouse in an overheat situation, your engine became a thermonuclear meltdown machine.

IMO
you need to pull every piston (but specifically 6 and 5) outta there and find out where those flecs came from and if the pistons are still usable.
And you need to check your camlobes. If the intakes are going down, this will decrease your ICA, increase your Dcr, and subsequent cylinder pressures. And it might only take a few degrees in an already high Scr engine.
And in the end you are gonna need to know exactly what your timing system is doing. Your carb seems fat but close to right, but impossible to tell for sure with those plugs.
This is my opinion.
Feel free to invite others to chime in. These kinds of failures, we all like to try and figure out.

Now, just to ease your pain, I took a a bunch of metal off the top of my 1977 block,and head, totaling about .100; and installed it into my car in 1994, with less cam than you have,(and therefore an even earlier ICA), and it is still motoring around today, with nearly half a million kilometers accumulated (that's over 300,000 miles to my American friends).
It's only money........