my advice is to go for the 68. If you already have a 80 l board i think a 68 will add more fun to your sailingexperience because of the much more differenciated feel between rides.
The kodes are freestylewaves so don't underestimate their speed. They're meant to move, act and react to harsh pushes and pops but not to smooth, more delicate weight displacements like a hardcorewave would. Off coure, when you're sailing 4.2, the difference is very small.
If you want a 2 board quiver for 3.7-6.2 my advice would be 68 for 3.7 up to 5.0 (max) and 5.0 and up you should be able to pull off on a 90 L freestylewave like board. Me personally, i sail everything on 90 L from 4.2 up to 6.2. Granted, its not comfortable but i'm a big guy and i can manage it, especially since i sail a hardcorewave board. I'd replace it with something smaller but i don't have the money :-p
I'm not going to tell you exactly which board to buy. You're on a SB site, so you can check all the wave and freestylewave boards they have. But all boardmanufacturers have freestylewave boards, all with different measurements and volumes. Inform yourself on whats on the marked before you buy. I mean, a cm in width may seem irrelevant but sometimes it's just what you need. Same for length.
hope this helps (nah, it probalby gave you more questions :-p)
Haiko, AKA crazychemical