The ones I've seen have a York compressor, so any A/C company should be able to fix you up.

Fantastic, I don't really mind that, I can always keep the vent door closed when I'm not using it. But hell, if i'm finally going to fix this, even though I don't drive it in the winter, I might as well get me a new AC compressor. The one I have seized up. I don't suppose you know where I can get on of those? :)

Yes, if you want 120 degree hot air coming in all the time, just remove the valve and run a hose directly to the core.

You can still buy new 1200 heater cocks, so maybe they still have B310 units avaiable.

In mine I believe the valve is actually corroded shut. I suppose the whole heater core may not even be bad. I wonder if I can just cut that valve out all together?

If its only for a dimister buy 1 off eBay.they work ok can be hot or cold and only electric meaning no water in the cabin :)

Put the lever to DEFROST and turn on the fan. if that doesn't work then something is broken

I fixed a car Sunday where that didn't work. Turned out the ventilation system was full of water.

im trying to find a way to demist the windscreen in my 79 b310 too.

yes that does work, although it doesn't shed as much heat. And it won't be on all the time as the valvecock is what controls the flow.

I was also once told by someone that you could fashion a heater core just by using a large diameter pipe and making fittings to connect to the water lines. Ya ever tried anything like that? Obviously the heater would pretty much always be going.

I doubt if 310 core would work. Heater cores are usually different for every car series.

Your best bet is getting one out of a 210. There are some in wrecking yards from time to time. Helman in Sydney, Ohio has one for $30.