I think it's largely a matter of technique, but you should definitely not think the board is to advanced for you. I'm sure you will improve by the day.
The "special" thing with the 76 (and its siblings) is that you have a lot of fin area on it, but most of it is placed very far forwards. This means that unlike on a single fin, you can not load the back foot up as much. The key is hence to try to stand a bit more upright over the board and not push the back foot. Then, the board can track upwind in an astonishing way. To get into this "tracing mode" is easier the faster you go, so a major no-no is pushing upwind too early, before getting up to a bit of speed. With some practice, it will track upwind at slow speeds too, but then it is even more important not to push the back foot.
If you're into tuning, a bit longer harness lines might help you get more upright and get off the back foot.
Using the boards as a single fins might help you get a bit better feel for the hull itself. Another option would be combining the (super soft and flexy) 15cm front fins with a real center fin around 16-17cm and (for starters) move it back in the box a bit. This will shift the center of efort of the whole fin cluster back and make the board react more "normal". Such a setup will surely help you go very well upwind with less technique change. Then you can gradually move the front fin forwards and later move back to a quad setup to get more looseness out of the board.
Tell us how things proceeds.