It would be good to know (maybe you already do; I don't) if the BB on that bike is French-threaded. When I converted my Gitane, I had to find a longer Italian spindle to make it work with the thin French cups. This may complicate your conversion.

FWIW, I never had much trouble with the cottered cranks. The key as far as I'm concerned is to use a press of some sort in combination with heat (propane torch) to get the cotters out intact, then clean everything thoroughly and slather the cotters with anti-seize compound when you reinstall. You'll never have a problem getting them out again. I only converted that bike because I came across a crankset I really wanted to use, and that bike was the one that suited it best.