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1/4" from the bottom of the fenders and a 1/2" really isn't that much, there is that much adjustment in the bumpers by just loosening them and repositioning the brackets. You could also slip some shims under the cab mounts to raise it up a bit.
Does it lean worse with weight in the bed?
The easiest way to tell if you have a fatigued spring pack is to place a load in the bed. The weak side will layout at a more progressive rate when you add weight centered at the rear of the bed. You should equalize tire pressures and on level ground measure the distance from the center of the spring pack to the frame. If you jack up the low side enough to equalize the measurement and the body levels out then it's probably fatigued springs causing the lean. Do the same at the front from the edge of the spring pocket to control arm. Check for rot, body mounts, collision damage. The measurement points on the frame are in the service manual too.
The front springs are brand new, so are you saying i should swap the front coils to see if one is weaker than the other?
There is no such thing as Chevy Lean.