ok I might be able to help you out here. I just finished a 56 biscuit with almost exactly the same dims. If you are using probox, probox larry gave me these dims for quad placement. Top 2 boxes 12 inches from tail and bottom 2 fins 7 in from tail. (both measurements from back of the box) all fins 1.25 in. In from the rail. Have a spread of 3/4 in. In between the boxes.IMPORTANT: have the back 2 fins parallel to the front 2. I toed the fins in 1/4in. And it seems to work well. You might wanna modify the lengths from the tail cuz u made a 54. Search biscuit quad placement if you want proboxlarrys version of this. Good luck!!!