Why do you want to install a threadless stem? The Peugeot is not a bad bike but certainly not high-end, but given that the bike was designed for a quill stem, why not just stick with a quill stem? If removable faceplate is your desire, you can get quill stems with removable faceplates. The only reason I can see to convert is if you really want to use a bar with 31.8mm clamp diameter.

Peugeots of the late 70s and early 80s typically had handlebars with a 25.4mm clamp diameter, and given that the quill may be weird size, I would just stick with the quill stem that's original to the bike unless you want to get the bars further away or closer to you (horizontal adjustment). I have the basic Nitto B115 25.4mm clamp diameter road bars on two of my bikes (both rebuilds of late-70s or early-80s frames), and I'm a big guy.