Depends on how bad the heater core is. The last bunch i pulled was in great shape on the inside. The antifreeze was still in the cores and hardly any corrosion or deposits. I just ran diswashing soap through them for 24 hours. I used the same pump I use for water cooling. Also cleaned it out in the process.
Now the fins are a different problem. They got a lot more crap on them that you can see from the out side. I sandblasted the outside with fine grid sand (same stuff i use for sand etching glass) and then washed it off with water, let it dry for the night and then soaked it in MEK. Then rinsed it off and let it soak in soapy water for another 12 hours.