Hi Marlin.
I think you'll find that there is metal, and certainly at the lower rear of the front arches...
I removed a section of the "Mud" insert and had to trim back about 1 1/2 inches of metal...not structural, just bracing between outer skin and body.
Having changed to 35's some 5 months ago...I am still cutting metal and plastic everytime I go off road and flex/compress more than last time.
(I believe Dub found the same when he went to 35's..)
Not so much at back arches...but certainly front arches, front and rear.
As for clearance at lower front of rear arches...
As long as the rear wheel clears the front "at rest"..should be OK (ish!) when flexed as the whole deal swings back as it compresses.
IMO, back of rear arches and inside top & lip of all arches is gonna get interesting with those 40's..
FWIW..I envisage some serious cut and hack on the ORW at Moab next time..
... when I push it to the max.
jo