Rebuilt 340 really difficult to turn over

This is what i would do:
Buy yourself a gasket kit and tear it down. At least pop off the heads , intake, and pan.
If the bores are rusty, it has to come apart and the bores properly cleaned and reinstall the pistons etc. after they are cleaned up too. Pay attention to the bores if they are rusty, and look closely for pits.
The crank should turn freely with the pistons out, if not, take the thing apart and check all clearances and correct where needed.
Don't forget to check the valves for being stuck especially if the bores are rusty.
Years ago, i had a good running 340 i bought off a friend that ha 10,000 miles on it and i stored it in a shed for about a year sealed and covered.
When i went to use it, it was stuck.
I ended up pulling it apart to find #4 cylinder was rusty. Long story short, i ended up having to sleeve the cylinder and had the heads redone with new rings and a clean up hone job on the cylinders to clean all of the crap and rust out.