If you pull the heads check the 3 cooling slots in the top of the head gaskets....they are like .188 wide on late gaskets, .455 on early stuff. Dirt in block, head gasket issue, blocked water passages could be your problem since you have no flow. Thin slots will not let dirt thru. Google bigblock mopar head gasket cooling slot. I have been making them .250 on everything I do with no issue, right or wrong.