the shock being closer to 1 frame rail then the other wont matter. as long as you can get your upper bolts in and out. to find the center of the car go off the wheel well openings. take you some plumb bobs on string and tape them to the over the center of each wheel opening. all cars have the engine offset to the passenger side of the car. that is why you got what you did measuring off the axle flanges. if you have not welded in any of your brakets take your top mounts and put them off each frame rail to where you can get bolts in and out. what type of shocks are you using?