your mechanic must not have much experience with turbo cars
no need to remove header for passenger upper just pop the bolts out of the frame and the arm will lift right out , its easier with dp out of the way but it can be done , with aftermarket down pipes it even easier .
youll want to get a GM control arm bushing removal kit to change out the bushings once the arm is off the car , might want to check on the balljoints while your at it