Need help! 285/50/20 or 285/55/20 fit non lifted stock 05 2500Hd with no rubbing?

[Tookie]

I have a 2005 2500HD duramax with 285/75/16 Tires on stock rims and on 16x8 aftermarket. No rubbing on stock rims but the after market rubbed a little I had to trim some of plastic air damn and front of inner fender and took care of most rubbing other than back of inner fender on hard turns. I havnt and dont want to lift level or crank torsion bars on truck but I would like some 20" rims and tires for it. I have installed some 3" beltech Lowering shackles which lowered the rear 2". What size 20" Rims and Tires can I run without any rubbing? I would like to have 285/50/20 or 285/55/20 Tires. How wide of a rim would I need and what backspacking offset do I need to run. I would rather not have the rims sticking out a whole lot of the truck but some is good. I hate how the front of these trucks sit wider than the rear. Any help will b appreciated! THANKs!