Originally Posted by
JohnDThompson
Which version of Simplex dropout do you have? The old, unthreaded 9mm hanger can be tapped to take a 10mm threaded derailleur mounting bolt, but you may need to file a stop tab to hold the derailleur body in the proper orientation. Otherwise, Simplex derailleurs that would work without any dropout modification can still be found on e.g. eBay. Later Simplex dropouts were threaded and incorporated stop tabs to accommodate non-Simplex derailleurs:
This doesn't apply here. Your Peugeot is low end and uses a claw hanger. Any RD with a claw will work. The Simplex claw won't work with anything but some Simplex derailers.
I doubt if you will be able to use much of your 105 group on that frame. The headset and seat post won't fit. The brakes almost cetainly don't have enough reach. A Phil Wood BB or even tapping the BB wil cost more than the frame is worth. I'd look for a more appropriate frame.