Update on my adventures with the motor:
It's definitely not the same as the radiator fan motor, which looks similar but is longer and narrower.
In searching for a replacement, I asked Ken at Dave Bean if anything was available. He said no, but that one other car used the same motor, and he knew a guy in San Diego who has a few motors, and that he'd get back with me after asking that guy if he may put me in touch with him. Turns out the other car is the Cortina Mk II, and the guy sold me a fan motor for what seemed like a reasonable price.
After some cleaning it up, I tested the motor by itself at first then installed in the heater and found it to be good and strong, moved a decent amount of air- so then I tried to re-install the heater. Key word,
tried. For some reason I couldn't get the heater to go quite all the way in. I pulled it away to check for something obstructing things, and noticed a scratch-mark on the end of the motor housing, which seemed to correspond to the location of a screw that comes into the space the motor occupies. Was this motor a different size?
Out the motor came for some comparison. Here's what I found:
The black motor at top is the bad original one, the grey motor at bottom is the Cortina Mk II motor. Very similar, but as you can see, it is longer overall. Also the mounting flange is about 1/8" closer to the output-end, and the distance from the flange to the 'hump' at the end is almost a half inch longer. I'm guessing that the extra size makes it more powerful than the original motor, and since this motor works so well, I'm thinking there's got to be way to make it fit. I'm already working on some ideas. First, that screw that interfered can be relocated as it only holds a little mounting clip. After that, maybe with some spacers and extra weatherstripping I can back the heater off slightly. I hope it works, because this business with the motor has put me a few weeks behind where I wanted to be with getting the dashboard back in. Wish me luck!