That's a tough call marclucrey. On one hand, Q76 and Q71 seem rather close, but on the other hand the Q66 is indeed a small board. At least for me, the limit of the Q66 is not planing, speed upwind or gliding through lulls (I'm 72). If feels very efficient even with a 4.5 or 5.0. Instead, it is the small width that simply becomes a problem whenever you are not keeping speed up in the wave riding. At only 53cms, it need a lot of drive to stay loose and agile. But when you have the conditions for it, it really makes the high wind sailing more comfortable. So in a way, it's a luxury board.
The Q71 with smaller fins comes a long way to make high wind sailing comfortable too though. If you sail comfortable with 4.3 in the Q76, I think you can handle 3.5 on the Q71. I only tried it down to 4.0 so far, but that no problem. Even thought the Q76 and 71 seem close, I think they in practice complement each other very well. When the 76 start to be just that little bit cumbersome in the wave riding, the 71 is well into its range. So you'll get a perfect overlap that will enable you to choose board size depending on wave direction, wave type etc, not only sail size. Use the smaller board and a bigger sail to ride really powered up but still in control or the bigger board and a smaller sail to still get around easy but also be able to milk the most out of the wave riding.
So it comes down to if you want to invest in having the perfect too for the windiest of days or to have a more balanced quiver which gives you the ability to fine tune the "feel" you want for the day by choosing board size.
Most sailors in fact tend to stay quite happy on 70+ boards even in very windy conditions. Check out the video below. Most sailors are light weight but I still think I'm the only on on a sub 70 liter board. This IS the type of day when the Quad 66 really shines though (3.5 sailing). So if you value the best possible performance in that kind of sailing, the 66 is worth considering, for sure.