I'll keep it and try to arrange something to be able to put a W neck, something can be done according to my instrument maker without modifying the body.
My instrument maker (on the right in the first picture) :
http://viewmorepics.myspace.com/index.cfm?fuseaction=viewImage&friendID=235868586&albumID=386573&imageID=22432848
The only problems : pickups will stay at their current places and the W neck heel will run a bit "out" of the body, but not too much (the old bridge mounting holes and strings-thru-body holes will still be hidden by the bridge afterwards).
The difference in length between both necks bottoms is about 2,5 to 3 cm.
Anyway, he's going to check that deeper, let's don't ruin a correct body for something worse...But I'm ready to take that risk.
If that works, I'll try to refret the old F neck myself. I've got a lot of time to think about that...