It depends alot on which weed fin you have, but it seems to me that you are a bit "over finned" with a 39 and 40 cm with a 6.5 m2 rig.
I've used Lessacher Design 32 and 36 cm fins with good success on a lot of boards, and I've used Tangent Dynamics Reaper weed fins in 39 cm/ 35 cm and 32 cm with good results as well.
What weed fin do you have that's 40 cm along the leading edge, and what is the angle of the LE of the fin..... 45 deg. 50 deg... progressive rake....?
Weed fins in general aren't the best for early planing, but the Tangent Dynamics Reaper and the True Ames Shallow Water Weed (with progressive rake LE angles) are amoung the best as you lose only a little performance when compared to a vertical fin.
Hope this helps,