RE: new wave boards
I'm your weight and also sail quite a lot in the sort of conditions you describe. First I would say that if you like the feel of the EVO 74, I think its best to stay with an EVO setup instead of a mixed Acid/EVO quiver. Personally, I'm perfectly comfortable with an EVO 70 all the way down to powered up 3.5 sailing and if you're used to riding the 74, I think you will on one hand allready be used to the EVO feel and able to take full advantage of the big range and on the other hand be surprised with how much easier the 70 is in lots of wind and in faster waves than the 74.
The 80 is in a way a bigger version of the 70 and I think its a perfect light wind complement. It's quite a lot of board for a 70 kg guy, but at the same time extremely effective in slower waves and lighter winds. So, the 70+80 is pretty much the perfect quiver. They are definitely not to close.
Personally, I think the sweet spot for the 70 at my weight (67 kg today after a hard hike on skis in the alps) is 4.0-5.0 and its really good also with a 3.5 or a 5.3. My sweet spot on the 80 is 5.3-5.9 and its good also with a 5.0 and a 6.3. Even if you don't use bigger than 5.2, the 80 will still be a nice addition to a EVO 70 when its marginal winds and onshore or if you want to ride slow waves or just need the float. So, I don't think you can go wrong with the 70+80.