I don't know that i could run 35s with stock keys, I think they'll rub frame at full turn but after i get new 15x10 steelies im gonna go with rear blocks and 35s, they should clear front with those, 2.5in backspacing.
Factory gmt400 keys can be cranked until the upper control arm touches the droop stop... (You do not want to do this) if your factory key will not crank your truck that high then your torsion bars are worn and should be replaced if you do not want to replace them, then you could use lift keys... But it's not the right way to do it. If you want to run 35's on 15x10's you will need to have a 3" body lift and you'll have to crank the torsion bars a lot.... And you'll most likely have to trim your bumper and fender. If you do crank then to the droop stop you can flip the upper ball joint to allow more suspension travel and to help with negative camber when you get an alignment. But a balljoint flip only works with certain wheels (with a low offset) and be prepared to replace balljoints, tie rod ends, idler arm and pitman arm often, because cranking that high wears them out, trust me I know!
Edit: just saw you want 2.5 backspacing... Those 15x10 on my truck are 3.5" I wouldn't go lower than that... Or you'll really have rubbing issues.
My truck with 3" body, keys, BJ flip,3" blocks and 35" bfg ats on 15x10's
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Same lift setup and 37" Boggers on 16x8's
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Both rubbed really bad!