Not really sure about the non planing problem. I also weigh 84 kilo's and don't have any problem with planing on this board.
fin position however, I would move forward. I always set up with my back fins about one cm from the front of the box.
Am sure you'll like that more.
Twin fins will loosen, in terms of early release when hitting lips to throw tail. But am not sure if it's going to help with early planing.