Well, I knew that I was taking a chance. The heater core could be worn enough, that the chemicals would cause a leak and I'd have to replace it antway.
Two days ago I flushed it with the pump on a drill. I used 1 pint of lime away mixed with 5 pints of water to dilute the solution a bit. One hose that went to the pump was the clear kind, so I could see that the solution was in fact flowing.
I filled the hose with a funnel until the solution was comming out the other hose. Then I knew it was full. I connected the lines to the pump and ran it for 1 minute, pause for 1 minute. I did this over and over for about 15 minutes.
Then with a funnel flushed the whole thing with clear water. Then flushed with a baking soda and water solution leaving it sit in the core for a few minutes.
Flushed the whole area with a baking soda solution also, to be sure no caustic chemical spilled on anything.
Hooked it all up and I now have great heat, with no leaks.
It worked great, but I knew I was taking the chance that I'd creat a leak..