I appreciate everyone's input on this. And I'm happy to say that my insurance company, State Farm, did right by me.
They did declare it a total loss -- but because they couldn't find replacement parts, not necessarily because the repair cost (in the hypothetical world where a replacement hatchback door could be found) was too high relative to the car's value.
To determine the value, they used NADA, rather than Blue Book or Edmunds. As others in this thread already noted, NADA gives the average retail for that year/model as $4800. But because my car was in good shape, State Farm actually valued it at six grand! And they're letting me buy it back for $800, which I could probably double if I wanted to part it out (I don't).
So I guess I'm in the market for a new old car. And I'll put my 510 in the garage until I can find a rear door for it.
I can't promise that everyone would have the same experience. It might have worked to my benefit that Datsuns are so rare in the midwest. And I also think that it helped that the body shop estimator was a big Nissan/Datsun fan. He put in a lot of effort to try to locate parts and I think that helped convince State Farm that the car had some real value.