The Goose Box length does not get added into any of the factors when determining clearance since it does not provide any additional clearance. Here is the formula that I'll walk you through:
(A + B) - C = Clearance
A = Distance from center of gooseneck ball to truck cab.
B = Length of Offset (in your case this is zero)
C = Width of trailer divided by 2 (you measure the nose of the trailer as that's what would potentially hit truck cab). view full answer...
The inside diameter of the Trailer Hub and Drum Assembly # AKHD-545-2-2K is right at the 7" mark. I went out to our warehouse and was able to confirm this for you. view full answer...