Been there, done that. Same thing happened to me. Engine had sit for 30 years, and when I turned the engine over 1/2 a turn everything locked up. Figured it was a valve since I had some movement. Tried spray penetrants and MMO, but still nothing. Finally, I decided to soak it down real nice with a 50/50 solution of diesel and ATF. Get a gallon of each, cause you'll need two gallons. Drain the crankcase before you start, cause that's where everything will drain. Fill the cylinders to the brim and wait two weeks. Then, try to turn it over. Hopefully it'll spin completely. Drain the crankcase and clear the manifolds. Worked for me.
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