Phillycop, the difference between the two kits is that the Readylift cost $160 more than the Auto Spring. No, not trying to be a smart elleck, but that's it. How Readylift sells those is beyond me.
As for the 305/50-20's, they will fit easily unless you use a wheel with a really wide widht and backspacing. Personally, I'd go with the 305/55-20's.

 

I'm not 100% sure but if you go 35's you may have to regear? I'm going 2" body 2" keys and running 33" tire for that reason? Help if i'm wrong!