Don't have an answer for you, but to say the stock bed is a better mount than the frame/rails of the truck, doesn't make sense to me. I have only used B&W hitches, so never used rails.
But, welcome to the forum.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired