When I had my engine out, I was still able to drain coolant out of the drain at the rear of the block, but on removing the core plugs found a bunch of crud halfway up the rear core plug (behind the bell housing). This was after filling the block with radiator flush and letting it set for a few days. I used a pressure washer and other implements (coat hanger) to clean it out, but I didn't remove the head as likely most of the crud was in the lower rear portion of the block where there's less flow. I expect mine to be fine, but the OP's may need a more severe treatment. Don't be afraid to poke around in the block with a coat hanger or other implement. The only thing you might damage is the oil line that runs between cylinders 3 and 4 up to the rocker arm shaft. And you won't be able to reach that from the drain hole. If the cast iron is thin enough that you could damage it with a coat hanger, that engine's a boat anchor at that point.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
