RE: wave boards?
First the small board: At 75 kilos you're kind of boarderline between the PA74 and 68. I'm 69 myself, and I think the PA74 is a better all round board, especially if the waves are not that good. That said, I remember when I first got on the 68 after having used a few differ 74s during the testing stage. It was like coming home and the test protocol reads "this is clearly my size". So, if you don't have to use it bigger than 5.0, want it to feel good also with a 3.3, have reasonably stable winds (or are a good schlogger) and since you say you like smaller boards, the PA68 indeed is the best choice in the Pure Acid range. The 68 has a fast an effective rocker, so it gets going early for its size. But it likes a bit more drive than the 74 to come alive on the wave.
As for the big board, I have not sailed the K79 myself, so I cant say how it will handle a 6.0. I have the K87 and have used it up to 6.3 and that really didn't feel like I was in the sail size limit for that board. With some experience in sailing small boards with bigger sail, I would guess a 6.0 on the K79 works out OK. The alternative would be to get a K87, but while the 87 is a very controllable and stable board, I would still say 4.0 is pushing it. And the 79 will "connect" much better with the PA68 anyway and be a great asset in the 5.0-5.3 range on semi-good wave days.
So, I think you had allready done your homework and picket out the best pair. PA74-K87 would be an option, but since all these baords are so effective (early planing, good upwind etc) and since you say you like to be powered up, the 68-79 seems like a better choice.
The K79 comes with a 24cm crossover fin. This one is great for B&J and might even handle a 6.0 since its kind of stiff and semi-thick profiled. The PA68 comes with a Drake Natural 21. The Naturals are remarably good wave fins but ride small. The 21 will easily take you down to 3.3. I suggest you complement these two fins with a Drake Natural 23 which will be perfect for 4.5-5.0 on the PA68 and 5.0-5.3 on the K79.