Gday there is a guy in Sweden putting 5.8 hulls on wider beams and putting a Tornado mast on with Tornado self tacking jib, which is sort of what your considering, if you want I will private message his facebook link to you, he's quite outgoing with his info, he currently has a widened 5.5. I'm guessing your looking at keeping your mast and maybe a F16 self tacking jib or Taipan 4.9 jib. Craig in Qld has put a new square top sail and infusion rudders on his 18 square and he says that has sorted the tacking issue. I haven't sailed my 18 square yet but hope to next month once I have the mast bits sorted, mines got a carbon 32ft mast and big square top sail and I'm hoping that will sort out the tacking. There is also a thread on catsailor.com which I revived a while ago and it has great info on spinnaker conversion etc. Good luck with your project as there are a couple more being revived at the moment and I have a mate looking for one to do up