FWIW, when I rebuilt my 216, I had to spend hours digging and flushing the crud out of the block. This was AFTER the engine machine shop had "cleaned" the block (they simply don't do as good a job as they used to due to environmental regs on the chemicals that are allowed). Anyway, I could not imagine getting all the crud/scale out of the block simply by flushing as described above. Flushing might help a currently running engine, but mine had been sitting for well over 20 years. The crud was more like mud than rust scale and very tightly entrenched. In the end, I got if fairly clean. Good luck.
