You have a few more things to worry about except the height.
First, unlike the SGC or AH-4 auto-couplers, the LDG will not tune an 8 foot whip 40 through 6. It's advertised range is up to 10:1 VSWR. It will tune it on 6 and 10, perhaps 12, and if the ground loss is high enough, on 15 as well, but that is about it.
Mounting an antenna atop a in-bed tool box is not ideal as it further increases the ground losses which are already high. You'd be better off mounting the whip to the bed rail by using a stake pocket or similar mount.
Here are a couple of suggestions.
I believe it was in the April 2004 issue of QST that Bob Lewis', AA4PB, article appeared. In his case, he was using an SGC couple. In that article, he explains several important considerations, not the least of which is the high voltage which appears at the base of the antenna in such applications. Bob also has a web site with some additional information worth reading. He also has links to a company which sells the mounts and insulators. http://www.ham-kits.com
You might want to visit my web site too. I cover how to properly bond, wire, and some other often over looked aspects of operating mobile. http://www.k0bg.com