Under which box beam? I can't see where you would need additional support as you have the braces on top. Maybe an angle from the left or right side (or both) where the hitch pins are connected to the arms and then back towards the pivot point, but that is the only "weak" spot I see. I will be curious if the single bolts holding the tines in place will work if you use this rake backwards. Not doubting, but I would have some concern as to how they will hold up.
Great job, and it looks like you have finished a great project. Thanks for the pics.