Originally Posted by Andy_K

I'm working with an old French bike myself (Gitane). The main pain aside from everything being an odd size (seat post, stem, handlebars, etc.) is that the bottom bracket is French threaded, so you pretty much need to keep it as is, at most switching out the spindle. This could make chainline difficult, but if you're keeping/redishing the rear wheel, that's a fixable problem, I think.