Here are some shots of the electronic heater valve I'm installing, and the valve with it's controller. It should work MUCH better than the standard valve which has vary poor heat control if it works at all. This valve (which appears to be a GM part) and controller are made for add-on rear A/C in van conversions. Bruce at Mac's radiator in Bellflower CA showed me the valve a couple of weeks ago. I went home to check to see if it would fit. It looked like it would so I went back and bought it.
The valve has 3/4" hose fittings. I epoxied a standard double ended flare fitting into the upper hose tube after turning the other end down (on my drill press using a file) to fit inside the tube.
I have replaced the stock metal heater pipes with flex stainless pipes which were easily bent to line up perfectly. In the early cars, the heater tubes run on top of the console which makes installing a modern radio impossible, so they had to go. The stainless pipes are 60" water heater gas hookup lines with 1/2" copper unions soldered on with JB Weld applied over just to make sure they don't leak. The unions are somewhat larger than 5/8", but 5/8" hose will fit over them. On the valve side I added a bead (14 GA wire wrapped around and soldered on) to make the 3/4" hose snug. I'll probably use braided stainless hose to join the pipes to the valve and heater. It could probably also be installed just using hoses. The valve is designed for mounting in the engine compartment, so that is an option as well.
The knob on the left is the potentiometer that sets the valve opening. It opens the flow very gradually giving precise control from full closed to wide open. The other knob is a three speed fan switch with an A/C on/off switch in the middle. The controller could probably be mounted alone with a bit of work. The wires connecting the controller to the valve are about 3 ft. long so placement should not be a problem.