No comment on what the OP should decide or Peugeot in general but rather about tubing.

The popular vote will go for the 531 tube frame, but that might be a false lead to follow. There's many framesets identified 531 but might only have sections of it. Then there's the sizing issue (small triangle vs.large) resulting in different stiffness, torsional twist and or loss of efficiency. Much depends on what you want. You have to consider the rider weight, rider strength, the bikes application (race, tour, etc.).
There's some flexi framesets built with R-531 that might not particulary suit a rider.

And another that may make one take a second look is in the weight. Frames built with 531 may not be lighter than perhaps a less regarded frameset / fork.

But thats also partially what makes it fun today in C&V. Its the affordability of framesets, trying them for awhile without any monetary loss. Another bonus is the fairly universal swap of components.

Try a slightly smaller size and compromise the adjustments with a longer stem and bar setup. I probably get the roll-eyes from purist but until they experience it, one should keep an open mind. The choice of vintage premium stems and quality bars can make a huge difference. I'm not just speaking of getting the dimmensions correct but also in their compliance / flexible qualities, crucial for upper body comfort and efficiency. Finally, get the right saddle for the job, a wheel / tire set-up and one can have quite the sweet machine.