Who is responsible for taxes during semi-trailer rebuilding?

We are sometimes asked to rebuild badly damaged customer-owned semi-trailers (consisting of a semi-trailer chassis and a semi-trailer body). During the rebuilding process, we remove all the body parts and fabricate a new body from all-new parts and materials supplied from our inventory. The rebuilt body is greater than 26,000 pounds GVWR. If the customer retains title to the semi-trailer throughout the rebuilding process, can we still be liable for any resulting tax on the rebuilt body under Internal Revenue Code Section 4051?