I emailed Steve today and he mentioned the same when I asked about backspacing with the 1.2 lift:
5” will rub the upper a-arm and 4” you will have to do a lot of trimming possibly even the metal on the fenders. What you want for the 285/75/16 or the 255/85/16 is a 16” wheel, the stock rim, 7” wide with 4.5” back spacing or a 8” wide rim with 4.5” back spacing.
Since I'm running 255/85/16s, I will confirm that 4" BS is better for that tire. I don't rub (with minor front fender trimming) and I chose 4" BS over (the stock) 4.65" BS to push them out and widen my track so my COG is lower. This setup is better for 255s, no question. Much more stable on and off road.
As for 285/75/16s, I took some quick measurements this morning on my truck and also based off my past experience of running 285s, I've come to the conlusion that both Steve from SS and myself are both correct and not so correct at the same time. Here's what I mean...
If you run 285/75/16s with 4.5" BS on 7 or 8" rims, you will be golden for the rears, but the fronts will be cutting it very close to the UCAs (most likely touching them if you run M/Ts). The difference in BS from stock will be about the width of a penny so tire brands will vary at least that much so unfortunately it is not a definitive answer. Lots of people have confirmed that they must use at least .25" wheel spacers with 4.65 (stock) BS and 285s. The only people I've heard of that run 285s on stock wheels and don't need any wheel spacers are running A/Ts.
If you go with 4" BS, it's the exact opposite as far as tire rubbing goes. You will be golden in the front (you will have about .5" between your front tires and UCAs), but you may rub on the rears now, depending on how much lift you put on. 4" BS may push the rear tires out a little bit too far and they might catch the inside lip of the upper fender (which can be trimmed).
I would say you have 4 choices:
1) 285 M/Ts, 4.5" BS, and just expect to have to run front wheel spacers (mostly depends on tire brand).
2) 285 A/Ts, 4.5" BS, and you
might be able to get away without wheel spacers, but you won't know for sure until they're installed.
3) 285 M/Ts or A/T's, 4" BS, probably some rear fender trimming (it will be cutting it close).
4) 255 M/Ts or A/Ts, 4" BS. No issues here. The tires are skinny enough to tuck into all the fender wells without rubbing.
I really like my 255s and I will not be going back to 285s if this makes any difference to you. I think this is the best setup hands down and I've had both to compare. Hope this helps you out.