Hotrod, thanks for the suggestion. Here's the oil passage on the engine block, where it meets the head.

Think I'm gonna first drain the oil (it's old oil anyway) and leave out the drain bolt. Then I'll fill the oil passage with carb cleaner. If there's a block, the oil passage should fill to the top and I shouldn't get anything coming out of the drain. Then I'll apply some air pressure and hopefully stuff will come out of the drain.

Suggestions?