Further manufacture: Truck-tractor conversion tractor taxable?

​​​​​​​One year ago we purchased a taxable truck from a dealer for use in our business. Now we need to convert the truck into a truck-tractor, which we intend to do ourselves by combining new components with components of the used truck. We have never sold any vehicles at retail. Can our conversion of the truck to a truck-tractor trigger tax even though the modifications are made solely for our own use and not in connection with any sale of the vehicle?