How much does the cub weigh? Nice detail on the floats!!

On a surpass 70, I would use a 13-6 or even a 14-6. It will give more static thrust for takeoffs.

Your configuration looks like it should take off just fine (although my preference would be to lower the tail in relation to the floats a couple of degrees). It could be you just need a little more experience on floats to acquire the finesse your cub requires. With the 70, you can't just fly through problems on brute power, so it will demand more flying skill, especially at low speeds.......but, once you get the hang of it, it will be worth it!!!