In case you're wondering how hard it is to get to the heater core, take a look. This is what you have to do to the dash to get that out. No fun.

Lesson to be learned: Don't bugger the heater lines.

 

I wonder if you could run some rubber heater hose all the way through the firewall to the heater core and hose clamp it to a good section of the hardline.

 

You could, I thought about that.. but, in a couple of years you might be taking the dash apart to replace a damn rubber heater hose. That would suck pretty bad.. I'm going with my leaky hardline was a fluke, since nobody else seems to have been down this rat hole...

InternetRoadKill has one from his parted truck, so I might get to drive my truck again soon

 

Yes please for your sake don't put any rubber lines back there - Aaaaaaaaaiiiiiiiiiiiiieeeeeeeeeeeeeeeeee

There is a reasonable solution though you wouln't have the lip on the outlet end ......................measure the size of the copper line and get some refrigeration soft copper and swedge the end , slip swedged end over cut end(where ever you cut it back to) and silver solder it and custom bend it before you solder it to relieve any stress on you o-rings/flange at core , if your worried about not having the lip then just make the end thru firewall a bit longer and double clamp ..............Waaaa Laaaa than you still have hard line and silver solder (refrigeration grade 10-15%) will never leak , heck you wouldn't have to add but 2-3" for that seeing your pic .
I would do it for you if you were in the neighborhood
Find a freind to do it for you in the HVAC Field , they will understand
Ship it to me I will set it up and send it back

 

I was going to suggest just that. If these are indeed nolonger available, why not just cut in a new piece of copper tubing? You should be able to get what you need from a local hardware store.