Joe - The picture below is taken from a 1953 Chevy Engineering Features document, but it gives you kind of an idea on how the cooling passages look in the cross section of the block.
They are shallow once you get inside the drain hole and you'll need something that can flex/bend to clear rust build up beyond where the hole opening is. Some folks have used a flexible cable like a thin plumbers snake to make those bends and follow the path of the passages.
Hope this helps.
