RE: Larger Fin for my F155
You may have one of the few F-155's in the USA with a std. Tuttle box.
Have you measured the depth of the fin box to ensure that you don'r really have a deep Tuttle box in your board?
Just becasue your board came supplied with a std. Tuttle root on the fin, does not mean that it's a std. Tuttle box.
So, measure the front and rear of the box and send me the numbers.
We can very quickly determine which box you have.
Yes, there's a good possibility of you "blowing out" the fin box with a 65-70 cm fin in a std. depth Tuttle box.
And, you probably won't find a fin that big with a std. Tuttle root.
So, you could modify a 65-70 cm fin to fit your std. Tuttle box (if for sure that's what you have) and plan on replacing the fin box (after you bolow it out) with a deep Tuttle fin box.
Let me know the depths of your fin box (from the bottom of the board) at the front and at the back. That will tell us which fin box you have.
Hope this helps,