That is a good looking setup- - - -is it home built or commercial? Do you plan to weld or bolt the adapters to the frame? Back when I was working with big rigs, we made it a practice NOT to weld or drill on the frame flanges to attach anything. It creates stress points that can lead to cracks and/or frame rail breakage. I'd suggest drilling holes in the vertical area of the frame and using sleeves over the bolts to position the mount brackets. That might be getting a little too picky, but that's how we mounted things like 5th. wheels, etc. on the big rigs.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!