French handlebar fit
I wouldn't try it. 

70's french stems aren't trustworthy, 80's don't seem much better and weakening a stem isn't exactly worth the risk anyway (I'd opt for sanding it rather than spreading it to remove the necessary material instead if I had to, but I'd recommend getting a new stem instead). You'll probably scratch up your nice new bars too.

Try to get the current stem out and replace it. If not going to come out, you may want to try sanding it, but I'd recommend just living with the existing bars.