I have used diesel followed by a penetrating fluid called Lloyds 'Moovit'. A squirt of Moovit through the spark plug hole every day for two weeks then I use a socket wrench to turn over the engine.
In my 1954 235.5cid engine I could used a 3/4" drive socket wrench and got the engine to turn over. (I inserted where a hand crank would attach, earlier truck engines have a different connection so this particular socket-wrench method will not work).
