[Marshall V. Daut]

In the 1980's when gasoline just started going south in quality, the BIG thing in the Model A clubs was to pour some magic elixir into the tanks to seal them. I'm not sure what the connection between antique car gas tanks and new blends of fuel was, but everybody and his brother were sealing their gas tanks. The idea smelled fishy to me, so I steadfastly refused to do that to my cars or friends'/customers' Model A's who had heard or read that this was the thing to do. I was concerned about down the line in a few years that whatever the Feds were calling gasoline might breakdown what worked well in 1980. I am no clairvoyant or wise man able to see into the future, but my gut feeling was: "Don't do it! You'll be sorry! Better living doesn't always come through chemistry!"
Well, the absolute garbage that's being passed off now as fuel for our cars is proving itself to be highly detrimental to rubber parts in the fuel system and to any older tank sealers that can't stand up to the new blends. We already know how Ethanol almost turns into corn syrup if left too long in a fuel system. I wonder if the OP's problem can't be traced back to old fuel that broke down and an older sealer letting loose? To have that much gooey crud inside the engine doesn't come from just one bad tank of gas. It had to have been accumulative. I should think that when the OP pulled off the cylinder head, there should have been one heck of a gooey mess. That would have been a sure sign that there's more of that stuff inside the engine, too.
Well, I'm sure we're all glad that the OP found the problem. Whether one person or everyone supplied the right answer doesn't matter. All the answers are things worth checking in case of future problems of similar nature among the readers. Good information for all of us to digest and store away for a rainy day.
Marshall