This is a mobile application. With that in mind, the metal of the vehicle is the ground plane. I use a 5/8 wave 2 meter antenna mounted on my headache rack on my truck. Also, I've run a ground strap down and attached to the frame. My opinion is that a 5/8 wave antenna is a great way to go. Just remember that for best omni-directional performance, the antenna should be placed near the center of the vehicle. I will be doing the same for my GMRS antenna.
73 de WRVQ648 / KC1KLA