GVWR doesn’t necessarily determine taxability

We make retail sales on a certain model van body. We have sold a lot of this body model over the past 18 months and have always installed it on trucks having a GVWR below 33,000 pounds. Therefore, we have never paid excise tax under Section 4051 on our prior sales of this body model. Recently, a customer requested that we install this body model on a chassis with a GVWR above 33,000 pounds. Would the sale of the body to that customer be taxable?