Thanks guys. I appreciate the responses. Still I think there must be some acid that would dissolve the stuff. I'm just pouring it in the water pump opening until it reaches the top and letting it run to the back (which from the angle of this engine and length I assume would be about an inch and a half maybe 2 inches deep at the rear of the engine. Since I can not replace that rear plug and since I don't know if was steel nor brass I don't want to threaten it with the wrong acid. The CLR did wash quite a bit of crud out of there. For this purpose I have a nylon barb fitting screwed into the drain hole. Maybe I need to repeat that and leave it for a week or two. Or maybe as Jerry said, muriatic acid is the thing and just pour it in there with no dilution. The side plugs are steel. I can check them easily with a magnet. Thanks again.
Jon

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end