I pull a gooseneck with a one ton flatbed. In certain twists the gussets on the neck hit the posts in back.

https://plus.google.com/u/1/photos/photo/112636930370126476644/6191993461958631618?icm=false

I had the aluminum flatbed built in Pennsylvania and they mounted the ball rather low. I called the manufacturer of the trailer. They suggested I notch out the gussets. I have done that since the picture was taken and that fixed my problem. The higher you can mount the ball, the less clearance problems you will have. If you have removable sides, you won't have any problems.