69 dodge dart

When I fit a rearend, I determine the pinion center by placing a 4" diameter ABS sewer pipe into the tunnel (4 feet is long enough) , then measuring from it to the springs to find actual driveshaft/pinion center. Do it on level ground with the old rear end still in place. You will find that the original pinion isn't centered to the tunnel; about half an inch to the passenger side.