We could use some better pics but this bike does not look to have carbolite tubing (which was Peugeot's name for high tensile steel). The bike looks to have a nice vitus fork and some decent quality frame tubing but I can't make out the tubing sticker.

Why do you want to replace the fork? This is unnecessary for a SS conversion. Nor is there any reason to swap out the BB for a SS conversion. The BBs for an older French bike are not that easy to find. I'd keep this really simple. I'd take off one of the chain rings. Be careful. You need the right tool to take off a stronglight crank and that is not easy to find. Then just pop off the rear derailleur and run the chain over whichever cog works best for you.

Btw, you are better off asking this question in the bike mechanics forum rather than C&V.