I've done this in the past, and didn't work well. Printed side art at Kinko's and spray mounted it to the sides. Problem is, you need a good adhesive, and a lot of those spray adhesives are rubber based, not acrylic. The rubber adhesives tend to start bubbling pretty quickly, and within weeks, it was peeling at the edges with large bulges throughout the surface.

If you can get it done free, might be worth it for some cheap, temporary art, but as cheap as some of the packages are at
mamemarquees.com, you might as well get something properly printed to begin with. They have done some stuff for me before, and they do a really nice job, and can print on various types of surfaces, and can print your custom art as well.
Also, I would not recommend putting plexi on the side of your cab to protect the art. Would look a bit odd and not worth the expense if you have a good print job with good materials anyway.
Good luck on your project!