Reloaded pics:
The heater in my 77 always blew hot regardless of where I put the temperature slider and I tracked the problem down to a faulty water cokk valve under the dash. I had the capillary style valve (http://www.classiczcars.com/forums/thread46448.html) and spent a little time looking into a direct replacement, but after spending that kind of "quality time" working under the dash, I decided that I never wanted to go through that again. So I changed over to a ubiquitous newer style water control valve which I relocated up in the engine compartment instead of under the dash.
I grabbed a bunch of different valves from the junkyard from various Honda and Toyota products, trying to find one that had the correct flow direction, had the lever on the proper side, and opened when the cable "pushed" instead of "pulled". I kinda lost track of which was which, but I believe the one I finally used was from a 2000 Acura TL.
Here's the valve I used. Flow direction is left to right when lever is on the side pictured, and it's CW to open again with lever as pictured. That means I can mount it on the return line from the heater core and it's push to open. Note that I also had to make a new control arm for the valve to get the ratios correct between between the travel distance of the control slider and the travel distance of the new valve:
I'm sure there are much simpler ways to mount a valve and connect to it but here's what I did. I made a mounting bracket for the valve which also included the provision for attaching the cable control:
Here's the whole thing painted, mounted, and plumbed in the engine compartment. I attached it to the mounting bracket for the brake lines and fed the control cable through an existing hole in the firewall. I believe this hole is where the vacuum line goes for the HVAC system on cars with A/C. The hole location was a little higher than optimum, but since I don't have A/C, this hole was unused which saved me the trouble of drilling a new hole only to find out that the whole project was a mistake:
It wasn't a mistake! It works great and will be a breeze to work on in the future if necessary.