Will they fit?

[Jman2136]

So I'm wanting to put a 6" lift on 04 sierra 2500hd and I want to run 20x12's wrapped with 35x12.50's but some people have told me that I won't be able to run anything bigger than a 20x9.  Was wondering if anybody had some info on that.  Can I run wheels and tires that size with the right offset or back spacing?  Or do I just need to do a little trimming?  Or is it really impossible?