Just to be clear, you want to replace the straight-down king pin box with an extended pin box, which gives you another 16-18" behind the truck cab. The risk is the strength of the pin box mount. An extended pin box puts a lot of torque on the mounting bolts, whereas the straight-down pin has none. The bolts can probably take it, but is the framing of the trailer overhang strong enough? If not, it will likely twist or welds will crack under the stress. Probably not right away, but over time. Or at some big big pothole.]
I have no knowledge of how well the front of your Alpenlite is constructed, so can't help you evaluate it. They built a sturdy trailer, but the engineers only planned for the straight-type pin box. I would be very skeptical of doing this.