I have a 2002 Ford F350 7.3L 2WD with no lift of any sort. I was thinking if 35x12 tires would fit my truck (minimal being 30x12's)?

 

So I can't speak for high flotation sizes but if you want to go do the math 285/75/R16 has always been the "How big can I go without a lift" on the first generation Super-Duty's. I would honestly stay with a 10 ply load range E tire having the weight of a PSD up front.

 

How about LT285/55/R20 on 20x9 Rims?

 

Depends on the offset and backspacing. Too much either way and you rub on the fender liner or the springs. 9" may be a bit wide to have a worry-free fit as my 8" wide rims don't leave much breathing room. Others may chime in with experience.

I have a 4wd and my 285/75-16 BFGs b-a-r-e-l-y fit without rubbing. I went from 16x7 to 16x8 wheels but with almost the same offset as OEM. I don't remember the offset number but it is slightly (-). I basically added 1/2" to each side while keeping the same load characteristics on the hub/knuckle assembly.

There are many that have gone 305mm wide on tires with stock height but they have to be careful of rubbing (can't turn the wheel all the way to lock). Personally I don't want to have to worry about it.

I also recommend E-rated tires as ccerniglia has. When you are talking about 35x12.50 sizes they are usually C-rated. Your 7.3L without any fluids weighs just over 1000lbs alone. The front end of mine weighs 4400lbs without driver (yours will weigh a little less since it's 2wd) and there's no way I'd go anything less than E-rated...even empty. JMHO