Having the short bed Ram truck. When I bought the truck I already had a 18K Reese slider, it has 10" of movement to the rear. I went with the adapter to fit the pucks to attach the Reese. I would not want to go back any further then 10" . I would think it all depends on the length of your pin box. Some have had to go as far as trimming the corners off the back of the pin box. I have towed two different fifth wheels with the same setup, never been a cab clearance ,but I have never tried 90 degrees either.
The easiest ,and cheapest thing to do is go with the adapter, using your existing hitch , does not sound like with your pin box placement the cab clearance is an issue, especially if you have a newer fifth wheel with the rounded corners. I have no cab clearance issues even in the tow position with the Reese being 2" forward of axle center.
As said by others ,its going to be a tailgate issue at some point sliding the hitch back, and that mostly as said depends on the length of that pin box. I can measure the length of my existing pin box which is a Lippert , and compare it with a friends MorRyde pin box which I had on my previous fifth wheel. My thinking is the MorRyde is a shorter pin box then this Lippert, but neither was a problem.