Don't bother with the re-threading unless you are desperate. Here's what I do on these particular Peugeots and their semi-troublesome bottom brackets. I go and find myself a Japanese made, french threaded bottom bracket. These were made by Sugino and Sakae, as well as the ever-present Shimano. The reason is simple: the Japanese made bottom brackets have a thicker wall than the Nervar made ones that were standard on your bicycle. They can accommodate a variety of spindles from Japanese cranksets, which greatly increases the number of cranks you can easily use. In addition, they tend to be cheaper $$$ wise than Euro French threaded stuff; if you want to spend money, find an old Stronglight bottom bracket for French bikes (I certainly wouldn't stick a Stronglight on a lower end ride like a UO/UE8. Save them for a PX10, or a Lejeune Professional, or a Rene Herse, the last of which you'd be insane to convert to a fixie anyhoo). You can buy the Japanese French cups from your LBS for about 55$ (they are still made), but I'd wait for some on ebuy.

It helps to have access to a Var 30 or like tool (ain't much like a Var 30), because, boy, did Peugeot stick on those BB fixed cups tight at the factory, and boy, do you want them on tight when you install one. That way you can avoid 'em coming loose, as well as the (weak) loctitie on the fixed cup solution.

As to the splined stuff, if Phil Wood is making splined BBs nowadays (I have no idea), he also makes French (and Swiss!) threaded mounting rings. You will spend beaucoup l'argent on such a project, sorry. Again, I wouldn't consider this an option on a humble UO/UE8. Go with the first suggestion.

You can also buy a cheapie sealed BB for use on frames with stripped BB shells, about 20$ at biketoolsetc.com. Never done this, mostly because I have a French threaded parts pile, but I have heard it works

I've done several conversions around French frames, so yeah, I know a bit. Ask away.