There were many changes from 1970 to 1971 when it comes to E Bodies. There must be an avenue for Chrysler to test the changes both with the car and with the assembly plant. These changes have to be set up in plant and the mix of the line workers job might have to be changed or work moved around from one worker to another. Looking at just the Hemi build. The Hemi Carbs used Manual Choke. Exhaust manifold were different on a Hemi with less parts lot of routing changes of the fuel system front to back. This is just a few things that are different and would effect all engine sizes. The Cars basically look similar from 1970 to 1971 but they are very different build and it kept changing over the year. Why would a Challenger in 1971 have three different front bumpers and two different back bumpers. Change continued over the year with the implication of Body panel changes. You have to test or maybe the change would not work.