Put me in the BT, DT file. My '91:
Those are 285/75 R16's.
My thots on this vehicle as a platform:
In these two years those headlights are a real drawback. I recommend these:
http://www.rallylights.com/hella/150mm.asp (only place I've ever seen a H4 conversion for these lights) with two caveats: 1) the City light feature is silly and the plastic bulb holder on mine 'burnt' due to my 55/100 bulbs which 'fogged' up the inside of the lens/reflector assembly, so they have been removed with extreme prejudice; 2) do the headlight relay deal and wire it such that both bulbs come on with the highs. Currently only the inners (lows) or outers (highs) light up at any one time.
That vintage Sub in a 3/4ton will still have a semi-floating rear axle. Find an older 3/4t pick-up or Sub donor and get the full-float 14 bolt rear axle. I personally don't see the need for a heavier front axle like a D60. I chase desert racers with this truck in addition to it's short bus uses. So far (2 years & counting) my 10 bolt is behaving itself and not showing any strain. I can see going to a D60
IF you plan to run a much larger tire.
The 8 lug outer stuff is a simple bolt-on. Get everything from the knuckles out (caliper bracket, spindle, wheel hub, & rotor). Your existing calipers will work
IF they are the single piston design. If they are the two piston design, then get the donor's calipers for cores too. For this tire size or near it, and assuming a 700R4 trans, given an option you want a set of 4.10 axles. I have 3.73's and by the time I get the engine at it's ideal rpm I'm well over most speed limits. I actually get better mileage at 70+ than I do a 65. Not the most ideal combination. At 4.10 with the 285's I'd be back to stock rpm vs mph.
If you have quad front shocks delete one pair and go to a better than OE shock. I use the more upright of the two mounting positions and the ride quality suffers a bit for it. If using a stock type shock (i.e not a desert racing type) I'd suggest getting a shock that will fit the more laid back mount position as I think it will ride better.
Looks like you have about a 4" set of springs under your Sub. Find the p/n in the Rancho catalog for the K5 Blazer pre-runner front springs. They are NOT in the applications listing. They are only in the p/n listing. These springs are so unlike any other Rancho offering (in quality features & in ride) that they will amaze you. They are a 7 leaf spring with tip sliders that results in a surprising low spring rate.
While on the suspension, I highly recommend looking at the rear springs. In particular look at the under-rider over-load springs. They usually are in contact with the main leaf pack at ride height. Do you notice that things in the back go "ka-thump" when going over speed bumps? These over-loads are the culprit. Take them out and turn the over so that their arch points down, and then re-install them. This will keep them in the system for towing etc. and will make the rear springs more progressive, giving you a far better ride quality.
FWIW: Even at this wheelbase (129") I still notice the rear Detroit, and I have no plans to remove it. It is a key feature to the truck being able to do what it does. The front will shortly be getting a used ARB that I found on a m/c site.
Man I love those square body Suburbans! I would love to find a 3/4 ton 4x4 with the diesel! Beefy! I'd throw a winch bumper from Buckstop on the front... 
