When using "flush" products, unless you loop the heater hoses so the mixture stays in the engine block, you might also end up needing a new hater core, but the radiator will always be "at risk" too.
If you knock out the rear core plugs and they appear thinner in the middle, the other ones are probably similar, so plan on doing them all and then be done with it. Don't forget about the cylinder heads, too.
Once all of this is done and flushed clean, be sure to put fresh coolant into the system and maintain it correctly. Some of the newer OEM coolants have chemicals which "seal" the metal in the system to prevent/better diminish corrosion, BUT they have to be compatible with an "open" system rather than the current OEM "closed" system (coolant recovery system, some pressurized). Might put the new coolant mixture in, then run it a month or so, then drain/replace it so that any residual "stuff" can be completely flushed out, for good measure.
CBODY67