Great idea to collect all the wisdom. I would like to add
1. that the Simplex Criterium derailleurs are very nice shifting. And they will last if cared for so don't believe the hype. I splurged on an IRD 13-24 5 speed FW on a 6207 hub with a shortened axel to fit the 122 spacing. My PX-10 was rather abused before I got it but the derailleurs had no serious cracks and I was careful removing and reinstalling them. They worked flawlessly for several thousand miles before I let it go to make room for a Raleigh Comp.
2. Mafac racers are awesome and 1/2 hoods are readily available from Rustines and Koolstop makes great brake pads for the Mafac holders.
3. It is not a big deal to sand down a regular stem to fit that french steerer.