Oh, figured you meant chemical spray or dip cabinet like the kinds engine rebuilders use to clean parts. If your talking about just a flush through the cooling system you might as well just put the block back together and flush the coolant and everything out in the car. Or if you really want to clean out the water jackets pull the freeze plugs and run a garden hose in there and clean out all that crud. That’s what I did when I cleaned the block in my 1966 Ford Mustang. I don’t think flushing out the block through the coolant hoses would have gotten this shit out. I originally was just going to replace the freeze plugs because one was rusted out and leaking but you can clearly see how much crud was in there. I suspect you might have the same situation on your hands so it would be good idea to pull those plugs and wash the block out. Here’s some photos of the shit in my engine.