You see now .... all the dirty washing is come out in the public eye
Just joking. It is an issue. I did a mid-50s French build a while ago and I will be truthful, the damn pedal thread is
the single element I can't fudge and move to and fro modern comfort and ancient bondage contraptions. Plus cotter-pins. Plus steel crank. I have previously rammed in early Look pedals with a breaker bar in steel cranks from the 1920s as an experiment - it sort of works. Then try to follow on with Shimano, and the whole thing is a no go. Re-tapping may be more available to you, but where I am in deep France that service ironically is long long long gone at the LBS 60km away. It is a nightmare.
The honourable thing is not to molest vintage parts. Re-sell them as-is. As seen above, there is a market.
Try to source some French cranks with the right taper and pedal thread and move on is a happy compromise.
TA and Stronglight often get mentioned, but still Nervar did some nice things too.
https://velo-orange.blogspot.com/2006/05/what-about-nervar.html
Failing that, just sack it all off, and go for some older Sugino. Sugino Mighty something or other. Sugino was most certainly used by the French bike industry, and they did a lot of OEM stuff to keep up the with the pace of sales of Pugs, Motobecance, Gitanes 10 speed racers - even with French pedal threads. Solida may have done something, but there later alloy stuff is nothing to write home about.
This must be - in my eyes - the greatest and oddest thing in the bike industry. 1985-ish Look thanks to Bernard Tapie had the clipless pedal produced by saving, oddly, a company producing ski stuff away from any mountain region in Burgundy. 1986 everyone wants them.
Never produced in a French thread. Tapie in a nutshell perhaps. Didn't give a stuff what was earlier if there was no future in it.