What fin boxes have you got in there? What brands can they take?
Ive got a 7'4 funboard in the quiver, and mine has got future fins, which Ive never had any problem with.
Ive had two set ups on there - first of all I had the 'F5 458' fins in there which made the board surf like a mini-mal, then I swapped to a **** *** set up (damn, cant remember the model. I can look for you if it turns out you can use future fins) which turned the board into more like a long shortboard
The difference between the two set ups is remarkably small when you see how much difference it makes. The only (visible) difference is that the second set are ever so slightly smaller and narrower, and are a little more 'splayed out' than the first set.