It sounds like you've got a rod or main bearing tightened up for some reason. Have you tried a pry bar on the flywheel teeth to see if you can get any movement at all? If a bearing has overlapped, you can sometimes turn the engine backwards to its normal rotation and free it up, but it looks like you're going to need to drop the oil pan and inspect all the bearings. Hope it's nothing serious, but I'm not too optimistic about this one! Is it a babbit-rod, or an insert-type bearing engine? From the year, I'm going to assume it's got insert bearings.
Jerry
