Gonna to have to measure the max height and width of your cab and make the template that size in whatever application your working in to create the side art, 300 dpi or higher depending on what your printer wants. Adding a little extra for bleed if the printer you are going to use requires it for custom work.
As for the curves, I'd suggest just getting the measurements as close as possible, and make sure that nothing important like text, logos or specific characters/items you want seen are close to those particular edges. I'm not good with measuring that kind thing so I'd come as close as possible and just leave some room for fudging.
