I remember from a while back my dad or my uncle had a radiator flushing tool that would hook up to an air compressor. It had a conical fitting to fit any radiator hose. I think you hooked it up to the lower hose, fill the water jacket up, and then hit it with repeated blasts of compressed air. It worked pretty well. The air agitated the water, and flushed the water out of the top hose. Maybe we ran it the other was as well by elevating the lower hose so you could fill the water jacket up.
I don't know if that's the same kind of flushing you are doing, but something like that may help loosen up the gudge. Probably have to find an old timer who still has a tool like that.
I think I'll borrow that tool next time I'm back home so I can flush my block out later this year.