I think the Nacra 5.5 is an excellent platform to upgrade. It was always a sweet boat.

If I had one and wanted to do the same, I would do the following:

1. Contact Randy Smyth at The Smyth Team or Jay Glaser at Glaser Sails to design a new fathead main and smaller jib. The geometry of the 5.5 fits well with current F-18 sail plans. The spinnaker design and shape could also follow F-18 trends.
2. The mast should work fine with those modifications.
3. The boat will be faster with the new mainsail and easier to sail in high wind.
4. I would consider going to a boomed rig with a floating clew similar to what has been done on F-18HT, A-Class, and some Tornados. You'll simply get a better looking mainsail with a boomed rig.

Hope this helps.

Bob Hodges
A-Class USA 230