Here's my thinking;
Water in the cylinder will flash to steam during the combustion event, then condense back to water in the exhaust system. During the time the water is changing phases it sucks a tremendous amount of heat out of the combustion event. You should have noticed this as a huge powerloss, having to drive pretty deep into the carb. And of course the large throttle opening would suck gas.
As the water level in the rad gets used up and goes out the tailpipe, the engine overheats, you see it on the gauge and pull over, and shut it off. Now the system pressure rises and blows water into the chamber, or chambers. You fill up the rad and hit the starter with accompanying hydro lock. But you get it started and drive off, only to have this repeat.
My thinking is that the hydrolock couldda pushed the gasket out, and came after the initial overheat. So in this scenario, if it is accurate, you gotta go looking for where the water was entering the chamber.
Yes it could still be the headgasket.... but then the water shouldda sprayed out all over the dog-house and the outside of the engine.
I know the pressure tests cost money, but if you assume it's just the headgasket and put it all back together,under this assumption, and it does it again, which costs more?
At least;
Lay the headgasket back onto the engine,and onto the head; and see if the water is able to sneak around the fire ring, where it is pushed out.
Oh I missed that the low ones were the blown ones, I assumed they were among the 1thru 4. Therefore if 1 thru 4 were as high as you say, then yes you are gonna need racegas, or an anti-detonant-injection, when running at or near WOT, but probably not at cruising, because your effective compression ratio may still be in the range of pumpgas. Detonation is your measuring stick, and is to be avoided at all cost. Detonation may have driven the gasket out. So you will have to go looking for the evidence
So most of post 10 may be meaningless