Not all fifth wheel frames are created equal and all trailers don't weigh the same. Some older units (possibly Duane's) are built similar to a GN trailer with the main frame rails running directly up and under the bedroom floor. Meaning they are spaced about 6 ft apart with little flex for the cross members. I had one of those light weight units years ago.
Taller fivers require a recessed floor with the main frame rails spaced almost 96 or 102 inches apart. So, there is more flex in the cross members. While the front crossmember is frequently beefy, the rear one is not.
Finally, the latest notched cap designs no longer have a solid front crossmember or the roughly 2 ft spacing between the cross members. Instead the sectional front member attaches to the main frame rails just a few inches ahead of the rear crossmember. This applies much more twisting and bending to the rear crossmember.
My point is that good results on another fiver, especially an older, lighter trailer is no guarantee of good results on another trailer, particularly, a newer, heavier unit.
To the OP, unless your flatbed truck is used extensively to haul GN equipment/farm/ranch trailers and you also plan on using the fiver frequently, there is no real benefit in using a GN adaptor. If the fiver is infrequently used, then store the fiver hitch or just leave it hanging from the trailer king pin. The conventional fifth wheel hitch is easier for hookup and unhooking.