We used to make hundreds of van bodies for Mitsubishi Fuso and Isuzu (among others- Budget was also one of our biggest customers). The Mitsubishi and Isuzu bodies were very simple, pretty much all the same. They came in three sizes- 14, 16 and 18 footers. Most took solid plywood for a liner.
Anyway, there is a way that the shooter could've been inside the truck with the roll up door closed, the driver in the cab and nobody would've needed to even get out to open the door. I doubt they did it this way, but it's something to think about- the Todco rollup door that the truck comes with has an adjustable top panel (you need to be able to adjust the lean at the top of the door- sometimes they bang the roof going up). You can remove the top rollers from the inside of the truck with a wrench and the top door panel will then fold and lay in towards the inside of the body.
This puts the shooter then in the position of having to be elevated up from the floor a good ways because the shortest door for that line would've still been 77 inches tall. With the panel laid down you can subtract perhaps another 10 inches- so 67 inches tall to the bottom of the opening. But if the shooter did have a little platform built (or just a simple ladder) he could've communicated with the driver with a cell phone or two-way, the driver could've spotted the target and positioned the truck in the right direction. When the driver gave him the "all clear" the shooter drops the top panel, takes his shot, the driver cranks up, the shooter folds the door panel back up into place and replaces the rollers. Within one second of the shot, nobody would even realize that the panel had ever been open. No damage to the truck and nobody sees anybody walking around the truck.
It's an idea.