Does the 75 percent rule for further manufacture apply to the truck chassis and truck body separately?

A customer asked us to repair their one-year-old truck. The truck body and chassis, when new, were taxable. The truck body is badly damaged and will require extensive repairs, while the truck chassis needs only minor repairs. We understand that if the cost of our repairs does not exceed a certain percent of the retail price of a similar new truck, then our repairs will not be treated as further manufacturing. In doing this calculation, should we treat the chassis and body as one unit or as separate units?