After a few months the Mercier restoration is finally over.
I finally opted for a triple Stronglight 99 crank (found one with a drilled small chainring) to work with a Simplex SX410 rd, shifting is fine and they can handle a wide gear range. A new bottom bracket was necessary, luckily the chosen spindle length was fine and the chainline came up quite right. The Campagnolo fd can handle the triple, although it uses all the possible extension on the outside to reach the big chainring. I left the original brake shoes, but braking is not optimal. I will probably need to make an exception to the vintage rule and use modern ones. Indeed, the bike is all period correct from the end of the ‘60s, excepting the wheels which are a few years younger. Final weight at around 24.5lbs is fine, not the lightest but still light for a 58-size Reynolds 3-tubes.
Here’s a picture of the “bijou”, hope you like it, and thanks again for all the advices.