Some years ago I watched an old time mechanic/restorer derusting and flushing an engine block. I think it was a Lincoln V something. He used an air hammer with a very flat bit/chisel in it and "rattled" the block in certain places to jar loose rust inside. Any of you ever done that?

No, and I don't intend to. All it takes is one thin spot in the casting and you'd be looking for a new block....

I've seen an old speedometer cable recommended, spun by an electric drill. That should knock a lot of crud out without resorting to a boil out tank.

I expected this thread to be about toilets.

You could use a Needle Scaler to clean a block.

I use a braided cable in a drill. It knocks a lot of stuff loose.

fill the thing with evaporust and let it sit for a week