If your block is full of crud - you need to get it all cleaned out. Remove the intake and exhaust manifold, knock out the freeze plugs on the side of the block and in the front under the water pump and get a garden hose with a good spray nozzle on it and a piece of wire coat hanger and wash it out. It's a very simple task - but you have to remove all of those parts in order to do it effectively. You cannot do it via the drain hole on the back of the block.
