I'd just rip all that stupid shifting stuff off your Peugeot, don't worry about filing anything off the frame yet. Get yourself a $100 fixed wheel, a cog, and a lockring and go ride. If you truly love it then drop the coin to get what you want. If you decide it sucks then throw the wheel on e-bay and you're only out a few dollars. I certainly like all the fixies that I see on e-bay that have "I found out riding fixie wasn't for me" in the description, but you've already got everything you need to go ride except for the rear wheel. You probably can't be riding this weekend (unless you have a LBS with a fixed wheel in stock) but you sure could be out there next weekend.