Rebuilt 340 really difficult to turn over

I would check the cylinders for rust as stated above.

Then I would also take the oil pan off and look at the crank and bearings. Make sure that they are not scored and are clean.

I've seen a speck of dirt behind or in a bearing lock up a crank. Check all of your main and rod bearings. Maybe remove one piston at a time from the crank to see if the engine starts to turn over easier. Then with all of the pistons removed from the crank see if it turns over and how easily. the crank should spin and keep spinning on an engine without any pistons attached to it. You don't have to remove the pistons, just push them to the top of the cylinder, but that may keep you from testing the crank turning resistance.

As you 'release' some of the pistons from the crank, see if one in particular makes more difference than the others.

If you have a rusty bore, remove the pistons and get a 4" wire wheel for a drill and use it to remove the rust on the bores. Then clean all of the debris out before reinstalling the pistons. I like to lube the bores with Casite Motor Honey or STP. That will keep the bores and bearings lubed enough for the engine to start and oil pressure to come up.

For the engine to not turn over, the bores are rusty, or the crank has some interference with the bearings.