I would be concerned with those white looking chunks coming loose and plugging the radiator.
can't remember if it was in the 60 or 40 forum but someone had an inline coolant filter between the thermostat housing and the radiator. might be a good idea.
remove the thermostat housing and see what it looks like inside the head
did you remove the block drain from the side of the block?
are you taking the radiator in to be rodded out?