if you buy the correct perches, they will fit tight to the axle tubes so they will only go on square to the tubes. Since your housing is not symmetrical, what I did was measure the overall length of the housing and marked center. Then you find the spring center to center measurement and divide that by 2. Measure off the center line on the housing out in either direction and you have the center line of the spring perches. Set your pinion angle and tack the perches in place. Hopefully you measured your pinion angle when the car was on the ground and with the weight of the car on the suspension so you can use that on the new set up. I believe stock is 5-6 degrees if I remember correctly.
