So, you have the Drake Natural 21 and 24 already for these boards?
Personally I think the Drake Natural fins are the best wave fins I've come across, and I've used many MFC models as well as other wave fins. The Drake fins have a HUGE speed range and one of the best things about them is how well they work in a quiver. On a given board, you can use them both big and small depending on what kind of performance you want from the board.
So, my advice for tuning the EVOs you have for more cross on conditions would simply be to use Drake naturals in bigger sizes, for example use the Drake 24 on the 70 for marginal winds. If you want even more fin for the E80, try the Drake Natural 25 or even 26. If you need something between the two original fins I would suggest a (2008 or 2009) Drake Natural 22 which will be a great all round size on the 70 and a grerat powered up size on the 80).
If you still want to go for MFC instead, I would look at the K-one. A good all round size for the 70 would be the 21 (or 21.5) and for the 80 a 23. Then you could add a 24 for big sails or conditions where you want more fin drive.