Closer inspection determined that the threads on the axle housing are not as good as I first thought. I have ordered a new nut and the thread file (the pitch is 16 tpi). Thanks for the suggestion, Kurt. I have a spare rear axle housing that I salvaged from a '72 Midget years ago, but the threads on its left side are also boogered (or "mommicked" as folks in eastern North Carolina say). At least, I have a housing that I can use for practice with the file before I tackle the one on the car.
I surmise that the left-hand threads and high torque on the axle nut is to ensure that the nut sufficiently clamps the inner race of the bearing so that it doesn't spin on the axle and the nut doesn't loosen with the friction due to tire rotation. But since the nut is clamped by the lock washer anyway, it seems making it a left hand thread only increases the probability that some idiot will strip it trying to get it off.