My info comes from a Ford service manual by Ford dated May 1 1950. You have to remember you are crushing a copper and asbestos gasket which had a direct influence on the heat transfer from the sparkplug shell to to the head. That is what people don't think about even today with the tapered seat plugs, if they are not tight enough the plug becomes a hotter plug then someone thought they installed. I don't know what they have substitued for the asbestos today,
I'm sure someone jumped on the plug makers to get it out of there.
I was surprised, the plugs in it seemed really loose. The plug box said something about tightening to either a 1/4 or 1/2 turn. Anyway got the new ones in and the engine runs even better now that it has new plugs and the original wires, yes I said original, are gone. All but 1 plug were still within spec, one was under .020, probably close to .010 rest were around 025.