Quick update.. will post pictures later.
The body lines mostly match up. The brackets are not even remotely similar.
The LMM bumper brackets have two ears which sit above and below the frame rail at the end and one bolt that goes vertical through them and the frame, along with the 2 side reinforcements.
The 2015+ brackets have are each 4 bolt flanges that bolt to a vertical plate on the front of the 2015+ frames, similar to how superduty bumpers have been since 1999.
I was able to flip the 2015+ brackets upside down and side to side to get a nice flat surface on which to weld some plates to make custom brackets. A little work with some jacks to get the leveling and spacing right and it came out pretty well.
I still have not made/adapted the side reinforcement brackets because I've been dealing with the cold weather and plowing.
The grille does not fit at all and because my grille was already pretty banged up and peeling I just cut it; the cut grille lines up OK but not perfect.
Long story short, it's not a bolt on mod, it comes out looking pretty good but not perfect, but if you have a severely mangled LMM front bumper and can find a 2015+ bumper for a decent price it's a good option.