We need to design an access ramp into a site with a 15.5' vertical drop to accomodate a 63' long lowboy trailer. We need some dimensions of tractor/trailer combinations depicting ground clearance, vertical transition and turning radii.
That is info you'll have to get from the trailer manufacturer. Most "low boy" trailers do not have sliding axles, so their positions will be fixed. The tractor rear tandem will also be fixed; however, most "5th wheels" have some limited adjustment, but not enough to affect the necessary grade change.
It will make a difference whether you are talking about a true lowboy trailer (used for heavy equipment hauling) or a drop center trailer (also used for equipment hauling, but can be used for container transport). A drop center trailer actually separates into two pieces and is driven onto from the front; whereas, a lowboy is accessed by rear drive on ramps.
A 63-foot trailer is a long trailer. Probably requires a special permit.