Fill the cylinders with Marvel Mystery oil and let it set. I've had good luck loosening up these motors, after pistons/rings soak for several days. With a BIG screw driver, get underneith and gently pry agains the flywheel ring gear, just a bit in each direction. One old 235 that's running now, would not turn over at all, had to disassemble completely and drive the pistions out. Was rebuldable and a good motor after all that.
Oh, BTW first check along the block behind the generator for cracks before you go grazy with the lube job. Have found many of these that were left full of water and had huge cracks all the way down the manifold side of the block.
