Hello,
Thanks to your advices, I made the final installation this week-end.
I padded the bottom bracket with 2 x 2mm rubber washers
IMG_2624_1.JPG
I fitted small rubber mounts on the top fixing points.
IMG_2619_1.JPG
The long nut welded on the chassis (on top of FF4) originally was an M6 nut, but I drilled it to a 6mm ID to get a "tube" rather than a threaded piece. The long bolt is slid in this tube, and is attached to the other long nut which makes a union with the rubber mount. A lock nut secures the whole thing (and allows vertical angle adjustment)
IMG_2620_1.JPG
Now that it is soft mounted, the radiator will need to be attached by a strap (hooked to a plastic bracket on the lower water collecting box and to FF1) so the radiator can not "jump" and leave its lower brackets.
Thanks again for the inspiration.