Kasses engine is bogus and not even close to being within the rules. Should have been thrown out.
Smokey Yunick would disagree.
It's not a deck plate, it's a REALLY thick head gasket.
That combustion chamber insert is something else too. I'd be curious to see how that is located and sealed.
My guess, is he bored/countersunk the chamber, only enough to properly locate the insert and stay out of water. On the insert, I’d guess it has two threaded holes on the backside. On the valve cover side of the head, two holes would be drilled to match the threaded holes on the insert, install two bolts/cap screws to hold the insert in place. Wouldn’t need to be very big bolts at all, the valves and combustion pressure would keep them put for the most part. After the inserts are in, machine you chambers and valve job.
If you hit water countersinking the chamber, I’d probably try the stuff you get from GM (little pucks) you put in coolant systems to fix small leaks