there's no adjustment in a standard sierra suspension. excess negative camber is generated when the car is lowered. you could get it out again by buying adjustable track control arms (TCA's) that can be shortened to bring the wheel more upright again. or a bodgers way would be to make a new or longer hole where the TCA inner mount goes through the engine cross member. a long hole is an ok idea if you set up the camber and then weld a thick washer in the right place on each side of the x-member.