I have an Anderson Ultimate for the gooseneck. However, if I remember from the videos correctly, the rail mounted version may be turned 180 degrees. I believe that would give you the extra distance you are asking about if you in fact need it.
In my short and recent experience, Anderson's customer support is great. Maybe give them a call after you experiment/measure, just to confirm.
You might even find a good distance with flipping the hitch 180 degrees (if allowed with the rail system, which I believe is) and reversing the coupler (which I was told was okay by Anderson and have read on here). I believe, with your set up you have a few combinations to test.
I pulled my 5er for the first time 2 weekends ago with the Anderson Ultimate and B&W gooseneck combo. If your experience is anything close to mine, you are going to be very pleased.
