I'm digging up this "old" thread just to update it. And because I'm very happy today.
First I want to thank you guys for your help, with special thanks to Kev' for his good advices (as always). Today, I could "repair" the fitting holes of my roll bar. As I said above, I enlarged them, and they were oval. Today, I finally decided to add some weld on the inner edges and grind them to recover nice and round holes. I delayed it because I was not sure if "I could do it". But it actually worked very well, and now, my roll bar fits perfectly!
So Thanks again to you guys for the help.
Here are a few pictures:
After I enlarged the holes two weeks ago.
Today I added weld on the inner edges. Grind flush, grind "round" (sorry, I can't find the English word for that)
All the holes are OK now and and the roll-bar fits !