Need Help w/ Lift and Tires

[mothertruckr]

So, Im trying to figure out what lift tire combo to go with and before anyone says anything about where the search bar is... I already have been doing a lot of research and still need help.

I have an 07 nnbs GMC Sierra 1500 4wd right now it has 275/60r20 factory 20" rims
Guy I bought it from said it had a 3.5" leveling kit? Looks to be more than that to me. heres some pics:







That block in the rear measures 3 inches and the truck looks level.
The tires it has on it have plenty of room...
the tires i want to get are the bfg KO2's in either the 35x12.5r20 (preferred cause its cheaper) or the 285/65r20... both are a 35 but the 285 is 1 inch narrower

So will the 35's fit?
Do I need bigger lift?
And longevity of everything, if my current set up is shortening the life of my cv axles for example, I would want to switch to something with better geometry.

on a side note the truck will almost always be road driven, Im in college and dont plan to tear up my only vehicle by going mudding, but I have always preferred the ability to go offroad when needed via storms or other scenarios. Where I currently live in florida it floods every spring and I have frequently had to drive through a foot deep, or more, of flooded roads to get out of the city and to higher ground... or like two years ago with my lifted duramax I spent half the day pulling out stuck and stranded vehicles. AND where I might be moving to next year can get a foot or so of snow on the regular. So I definitely want taller than factory height.

Thanks in advance for any help