French bike??, first make sure you got the right smaller quill size if the French bike has the smaller diameter French tubesets. French stems will also most likely have a smaller bar clamp ID that will require a bar of the correct diameter too. That is why a lot of handlebar end caps that came with bar tape sets were a very tight fit on French bike bars like the Phillippe Guidon bars that Peugeot had on may of their bikes in the 80's.
If that all checks out, then you can proceed with your install by loosening the top quill wedge bolt so that it slightly protrudes from the top of the stem and you can give it a tap with the mallet to loosen the stem from the steerer tube. If it won't loosen with the wedge disengaged, you might have a stem fused to the steel steerer tube with corrosion. hopefully a few harder taps with the mallet would break it free, but if it doesn't, you might have to resort to more drastic measures which could most likely destroy the old stem and if you are not careful, your fork assembly too. Your LBS should be familiar with these "alternative" methods as they most likely deal with stuck stems almost every day of the week.