I just used CLR to try to clean some crud out of the rear part of my 235. One of the warnings on the CLR bottle is "don't use on brass, copper or aluminum." Your head gasket could (like mine) be the copper sandwich type and your radiator is either brass/copper or your new radiator could be aluminum. Some of the new ones are. Muriatic acid and aluminum gives off hydrogen gas plus muriatic acid can and will dissolve aluminum.
I've been trying to figure out the best acid to use as a lower block cleaner (engine not running), but I haven't gotten much guidance yet. Hopefully somebody here knows the answer.
Good luck.
Jon
