My experience with the F-Type 158 (actually I used the '05 F-Type 148 and '04 Free Formula 138 (both 96 cm wide)) was always that a 70 cm fin was a little too big and tended to limit my top speed.
I also found that I could (at 170 lbs./77 Kg.) get the 96 cm wide boards going just as early as the F-Type 158.
If you are heavier, the FT 158 may be better for you. If not, I would suggest going with a smaller 66 cm fin.
I had a Deboichet Concept 66 cm that was simply awesome on the 96 cm wide F-Types.
I'd try a bit smaller fin (borrow or demo one if you can).
Also, at my lighter weight on the 96 cm wide boards I found that 9.1 m2 sails, on 100% carbon masts (light weight rigs here) got me going just as early as the 10.5 m2 rigs.
Maybe this is a function of my smaller size (5' 6") and lighter weight.
I'd try a smaller fin first, and see if it's the particular fin that you have that's the problem here.
It may be a very powerful fin, once you get it going, but it may not have flex characteristics that are working for you.
Also, how much are you "tuning" your V8.
Remember, for the earliest planing with the least pumping, you need to rig the V8 very full with less downhaul and outhaul than recommended. Keep the top of the sail a little tighter with less downhaul and get a good adjustable outhaul system so you can lean the sail out, once you get going.
Also, where are you running your footstraps?
Hope this helps,