I'll bet I spent a couple hours with a long screwdriver, punch, chisel, hammer, and a shop-vac cleaning the water jacket on my 235. There was no way any flushing was going to remove all the rust and dirt in my block. All the junk had built up in the back of the block around and behind #6 cyl. I had to work through the freeze plug holes, and the coolant passages on the top of the block. (engine was out of truck) Then I'd tip the block over a few times, and suck it out with the vaccuum. I know radiator flush wouldn't have completely cleaned mine. Maybe yours isn't that bad.

Keith
If it were easy they'd have grade schoolers doing it!