Adjust the stock torsion bar arms (IE: keys) to where you want them and then get an alignment.
If you put the "rip off" aftermarket keys in you're still going to have to get an alignment done.
No matter which way you go, the z height is going to be out of spec and the shop is going to know. When the gm ifs is lifted by "cranking" or "keying" the torsion bars, the camber goes waaayyy positive. Tires go like so : \-/
Z height is what they measure to know if the truck is at stock ride height. It's the measurement between the center axis of the the lower control arm pivot to the bottom of the steering knuckle. I know, that's a bit hard to visualize so here a couple pictures:
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As you can see in the first pic, there is no measurements at the torsion arm. It's adjusted however it's required to give the correct Z height.
Most shops won't crank the bars because it's a modification from stock. This means they are opening themselves up to litigation if something happens, like you drive off the road and flip your "ambulance chasing lawyer" can argue you lost control because they made the vehicle behave differently then OE by not adjusting it as per specifications. You can thank the millions of spineless assholes as ****** ambulance chasing lawyers looking for an easy buck for that....
If you show up with them already done and ask for them to adjust the camber and toe only, they don't have a problem with it as
THEY haven't adjusted anything away from stock.
Oh yeah, if they say they won't crank the torsion bars but they will install modified torsion bar arms they're just trying to suck more money out of your pocket. They're F'n crooks if this is what's happening because either way they're modifying the vehicle out of OE spec and thus open to litigation in the advent of an accident and a ****** ambulance chasing lawyer...
One last thing: depending on your truck, you may or may not have the camber adjustment slots on the upper control arms points. If you don't, you have to buy a kit from gm and have the slots cut to adjust the camber. It's mostly the early GMT400's that need the kit.