Thanks - I'll probably simply replace it. I have a few bikes in the shed that haven't been ridden in a long time. I'll see if any of them have a 170mm crank arm. That would be the cheapest route. (It's a lovely bike, a fully chrome 1977 Super Le Tour. It already has a hodge podge of parts, so there's no worry about keeping it in pristine vintage condition.) It's a question of do I spend the money for an approximately matching crank arm on Ebay or leave it looking funky with mismatched crank arms - which, admittedly, only I would ever notice.

I rode it around the neighborhood, and it doesn't seem any worse, but it's really not in my nature to trust something that really isn't right. I'm tempted to try to fit a helicoil in there, just for the experience, but I'm sure the helicoil kit to replace a pedal thread would amount to an expensive experiment, and since I've never done it before, it would probably be a failed experiment, and on top of that, I would have a helicoil kit I would never use again getting in the way of all my other tools.

I was mostly wondering if anyone out there had experience riding on a mostly stripped crank. Usually there's at least one person out there saying things like "it's not ideal, but I rode my bike with a crank like that for 87 years. If it screws in, it will stay there." Or, to the contrary, "I got two miles from home and the pedal came flying out of the crank right when I was on a twisty descent."