Cleaning out a cast iron block

First the freeze plugs in a 454 are too high in the block to let you see much less break up and remove what has settled in the bottom of the water jackets. Second removing and replacing freezer plugs in the confines of an engine bay with The exhaust manifold is very likely going to lead to leaks. I have even had customers crack cylinder walls trying to get freeze plugs out without knowing what they are doing. Your best bet short of disassembling the whole engine is to pull the hoses and pump and just blast a ton of water through it and hope you got most of the rust out..