If they are so well-clogged, your block is probably full of solid matter ("crap"). You will want to try to wash/clean out your block.
I have found it useful to remove the drain-cock on the lower right driver's side of the block, and then use a stiff copper wire (10 or 8 gauge) to loosen up solid deposits around the rear of the block (often flushing from various angles).
Use that stiff copper wire from the front - the holes/opening for the water pump. Try to work the wire from both sides, all the way to the back of the block. Flush often from those holes.
This will be messy.
In my opinion, I think the clogging you describe is why many people have overheating 216/235/261 engines.
