I had a 39 Plymouth coupe, the flat 6 in it was locked up big time when I bought it.

Tried all the mentioned tricks, nothing worked, first time that ever happened.

Pulled the pan and the bore's looked good.

Pulled the head and the bores all looked good, except for one bore where the water in the cooling jacket corroded thru the cylinder wall and that bit of rust was what locked it up.

Scrap metal then.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.