Use a very fine line to mark...
Use a very fine line to mark each intersection. Find a marker with two ends, one fine to mark the points and the other wider to label each point. Double-check each measurement to eliminate mistakes.
You will then have two points at each end of the car to use to develop a line that is at a right angle to the centerline of the car. Snap a chalk line or stretch a string over each of the centerline marks at each end of the car, forming a line. Make the line long enough to extend past the rear of the car. If you use a string, you can hold the string in place with spare lead weights or concrete blocks. That way, you can stretch the string tight and the weights will hold it firm.
Make a cross mark on the "centerline" somewhere behind the rear of the car.
Measure up the line toward the front of the car 96 inches (8 feet) and make another mark on the line. Again, you can use masking tape to mark on. I always cut 1 foot from the end of the tape so I can make an accurate measurement using the 1-foot mark and the 9-foot mark.
From the line you made in Step 2b, measure two distances, 10 feet from the front point and 6 feet from the rear point, to form an intersection on each side of the car. These two points will be used to construct a line that is exactly at a right angle to the centerline of the car.
Stretch a string or pop a chalk line over these two outer intersection points. The line that crosses the "centerline" should be directly over the rear 8-foot point on the "centerline." If it is not, recheck all of your measurements and re-establish the intersecting points that form the perpendicular line.
The triangles formed on the ground (if you haven't recognized it yet) are 3-4-5 right triangles whose sides have been doubled. In geometry, coincidentally, if a triangle has sides that measure 3 units, 4 units, and 5 units on each side, it is a right triangle. We just doubled the numbers, and we still have a right triangle. This is the easiest way to layout a perpendicular line.
Step 5 Drop a plumb line down off the rear of each of the axle caps on each side of the rear end and mark a point on the floor. Be sure to use the same position on each side of the rear hubs. The hubs themselves are machined and are best to use. Don't use the back of the axle tube, especially if it is the original cast type.
Use a good quality plumb bob...
Use a good quality plumb bob to project points from the chassis to the floor. Use a piece of masking tape to mark on. That way your floor remains clean. You will be doing this more than once, making sure the car is straight as time goes on, and you recheck it.
These measurements should be the same. Adjust the length of the trailing arms to square the rear end.
Lateral Rear End Location As well as making sure the rear end is perpendicular to the centerline of the car, we also want to make sure that it is positioned correctly between the rear framerails.
One of the handling adjustment methods that have been passed down through the years involves moving the rear end to one side or the other to change the handling balance. This is another unreliable and temporary fix that should not be necessary if all of the other chassis components are set up properly.
Alignment of the Right-Side Tire Contact Patches I always recommend that the rear wheels be positioned so that the two right-side tire contact patches line up. This is because in the turns most of the weight is on the right-side tires and the car will want to track inline with the right-side tires where they contact the ground. Regardless of what you believe, it is important to know how the right-rear tire tracks in relation to the right-front tire.
Measure from the "centerline" you have created to the center of the right-front and left-rear tires. Make sure the front tire/wheel assembly is positioned so that it is correct in relationship to the car's chassis as it would be at ride height, like we did with the rear end. Adjust the length of the panhard/J-bar to align the right-rear tire to your particular liking.
This whole process should take about an hour or so. That is very little effort to make sure your car will track correctly and misalignment will not interfere with your car's performance. Repeat this entire process often, especially after kissing the wall or being involved in a crash. Once you are convinced that your car is aligned, you can then concentrate on the other important aspects of your chassis setup.