This has been interesting to read - and now I'm riding the "crap" peugeot, loving it, and wondering even more what a 531 frame or such would feel like. I used the UO-8 frame and added alloy bits - wheels, bar, stem, crank, while keeping the MacFac racers. It's super light (as per my experience) and feels very sweet and responsive to ride.
So now I have to find a 531 to compare...Alas, more collecting.
Thanks for all the replies so far.
Bluespenny mike