A hammer and screwdriver around the cylinder walls is asking for trouble.
You don't know how thin the walls are (years of rusting) and you may crack them.
I had a 235 that was seriously blocked. No water out of the rear drain plug.
I could not get the area behind the plug clean enough for coolant to flow.
I'm sure that some of it was radiator stop-leak.
Had to remove the freeze plugs, water pump and drain plug(s) to get access.
Did the best I could with solid wires and long skinny screwdriver to clean the layers of crud from every opening I could find.
This worked good enough to get the coolant flowing again. Put everything back together with new head gasket.
Then did an engine flush with vinegar (or a store bought engine flush) until the water looked clean.
Final flush with water only.
Whatever was in the engine was also in the radiator. Took the radiator to the radiator shop for a boil.
The best solution would have been to pull the block and have it boiled-out at a rebuild shop.
