The problem is the 6" rim width. Metric to inch conversion is 25.4mm to 1", so a 235/75-16 is 9.2" wide on the bulge of the sidewall at the bottom of the tire with weight on it. That's already 50% wider than your rim. Your 285/75-16 is in reality about 12" wide on that bulge, going to a 315/75-16 puts you at 13" on your 6" wide rim.
The issues are two fold. First, is that the sidewalls are not in proper relation to the tread, meaning wear patterns, operating temperatures, ride quality and driving characteristics are all impacted. The second is that you need 1"+ between the bottom bulge on the duals to prevent sidewall rub, heat buildup, and potential failure during driving.
Do guys ignore all of those issues and run them anyway? Well, yeah, they do. And if you must have a taller tire, then do your best to get the tallest tire for a particular width, or rather the narrowest width for the height you want.
A better solution would be if a wheel manufacturer would start making 7.5" or 8" wide 17", 18", and 20" wheels for duals and build the correct offset into them to eliminate the spacers.
Sent by my right thumb!
The Current Truck (Oct-2013 to Present):
1999 (Early) F-350 Lariat, CCLB, SRW, Black/Tan
7.3L Powerstroke, 4R100, 4x4, 3.73, 37x13.50-18s, '05 Axle Swap, 6" Fabtech lift, GTP38 "D66" turbo upgrade, SCT 4-position chip with custom tuning by me, Straight Piped 4" down pipe through 5" dual stacks, N-Fab full length triple steps, Pro-Tech cab guard and tool box, louvered 5th wheel gate, Hella 500 Fog and Driving lamps, much more to come...
The one I'll miss, (Mar-2009 to Jan-2014):
2000 Excursion XLT 4x4
6.8L V-10, 4R100, 4.30, 285/75-18, 271whp / 388wtq
The one I won't, (Jul-2005 to Sep-2012):
2005 F-350 XLT, CCLB, SRW, 4x4 FX4
6.0L PSD, ZF-6S650, 3.73, 35x12.5-18, 4.5" Lift