Hi Willy -
My “le tube de retour” is like Rene’s.
Mine has a short soft line (with clamps) at the radiator and the main part of the tube is a hardline.
It was much easier to route with the engine out. It goes under the car to the rear bulkhead, and then over to the modified overflow tank.
There may be a few other sections that use soft rubber lines, but not many.
Good luck with your build!
Rocky