Velo Orange's French bottom bracket changes things a lot. French threading need no longer be feared -- at least not on the BB.

There are still potential problems as noted above. When I converted my Gitane (which had an ISO BB, BTW), the stem was irrevocably frozen to the steerer, but it was more or less where I would've wanted it anyway, so I overhauled the headset in place and just left it.

I'd say convert. Re-building an old bike builds character. I look at it like a Jedi making his own light saber, except that in this case he ends up with a kind of crappy lightsaber that he loves anyway and is much cooler than the crappy one he could have gotten from lightsabersdirect.com for a little less money.