Truck bodies and truck chassis - Separate tax determinations

​​​​​​​If a customer purchases a new truck body from us and asks us to install it on a used truck chassis, is the body taxable? Similarly, if a customer purchases a new truck chassis from us and asks us to install it on a used truck body, would the chassis be taxable? Does it matter whether the sale and installation of the new article occurs within six months of the purchase of the used article?