I used to have a 70 D200 with AC, and the heater core went bad in it. I tried to find one for it, with the same results as the OP. I took it out and took it to the place that did radiator work for me, and they ordered a new core,,,and just soldered my ends and pipes to it. Fit perfect and worked great.
The answer of take it to a radiator shop has been out there for a while. The part that surprises me is nobody ever says "where am I supposed to find one of those"? I mean, it's not like there's a lot of them left. Any way, this post made the DIY light go on: I know how to solder copper pipe in the context of doing minor residential repairs. How hard could it be to transfer those skills to this problem and leave the shop fees out of it?
Most of the heater core leaks I've seen were either on the core itself, or where the tubes connect to the core. If you have never soldered actual used heater cores or radiators before, I strongly suggest you practice on something that isn't very important. Fixing used heater cores and radiators that have had antifreeze run through them, and have been exposed to the elements for years is a different experience then soldering a copper pipe that you can easily clean. Then you need to devise a way to be able to leak test your work before, you have to assemble it into the box and reinstall it on the truck.
Years ago, the last time I took a heater core to a radiator shop, all they wanted to do was order a new one. According to them, their success rate at repairing old heater cores wasn't very high. This was an experienced radiator shop. There is no longer any radiator shop withing 40 miles of here, that I know of. I would be interested to know how much the suggested radiator shop charges for repairing a heater core, and I would like to know where they are getting a copper replacement core. Any new heater core I've seen in the last 3-4 years have all been aluminum. Gene
