Mark,
If the radiator will be out of the way, I'll suggest again that you remove the water pump and, through the opening in the front of the block, gouge out junk all the way to the rear of both sides of the block (using something like heavy gauge copper wire). Do this until you can spray at each rear side of the block and see the water coming back/out on the other side of the block. This is not so easy to do (from the front) on/in the higher/earlier/exterior water-pump blocks.
Then, after doing the above clean-out, again check all smaller coolant passages.
I have been able (with much work and patience) to clear out blocks this way (without removing block casting plugs).
