Dear GES
We make a 8 ft bridge and we can make it with either a 6 inch inside clearance of a 9" inside clearance. Attached is one that is made on my home layout. the large (' inside clearance has the added ability to hide a 3/4 pipe on the bottom for extra stiffness or if you want it a smooth surface it can be boxed and a duck or a LIFT OUT IS ALSO BETTER!