If you're closer to 70 (like I am) then the 71 will work a tad better with 5.6, yes. And I would get the 71 as my only wave board, but I also sail quite a lot of days with 4.0 and 3.5. For sailing here in Sweden where we mostly get kind of mushy waves I would still probably get a 76 if I was only gonna use it from 4.5 and up. At least for me, the 76 feels like one of those boards that hide its size (width) well.
I have experimented a bit with fins on the 76 (16/11, 15/12, 15/11 and also 16+14 and 16+16 as thruster setups), but not enough to really tell what happens in high wind. I don't think the "overpoweredness" on the wave at high speed comes so much from the fins on this board. But there will be smaller fins coming out (14 and 13 front fins, I think) so maybe you can wait for those if you feel the board is over finned in high wind.