Thank you all.
We already started doing just that. Or almost…
We copied and pasted the whole thing onto a proper sequence. FCP did leave a bunch of black holes all over the place, but overall, the sync seems to be kept with the music. We are starting to adjust each shot manually.
Of course, some of the intended flash frames have gone lost for they were probably interpolated frames that completely disapeared when in a 24p timeline.
Thanks again