Hi Ken,
sailed the Isonic 111 some weeks ago. Quite wide sailrange can be carried by this board. Had a 9,0 on. Would suggest 42 or 44 for 8,4, for the smaller sails from 6,5 - 7,5 36 to 40.
I use myself MFC. They are quite nice,.
But for sure the Deboichet fins are hard to beat, no discussions. Vector should work fine, too.
Good luck for your races,
Chris
www.chrispressler.com