One thing to note about the NASA info on LCGs: The NASA LCG uses a relatively high water flow rate (~.5gpm, and 1-2PSI of pressure drop) where the water temperature is varied to control the cooling. The LCG used in the current space suits is basically identical to the final Apollo version. They are very effective even at pretty high work loads and the cooling is pretty comfortable. But, they are a pain to maintain and very expensive to build. I was involved in the attempted commercial development of an LCG with similar performance, but we couldn't get the LCG and supporting equipment price low enough or the simplify the effort of using and maintaining the system enough to be worth the benefits.
The Med-Eng design uses a very low flow rate ( < .1gpm, and 10+ PSI of pressure drop) and ice cold water (as do every other commercially available system). This type of LCG can't pull very much heat from a person, basically just keeping up if you are sitting there and watching something. They probably won't cool you down if you start out hot, and won't be able to keep up if you are doing any sort of work. They are basically on or off. A lot of people find the ice cold water really uncomfortable.
I would set up the system quick and dirty and play with it before you spend much effort modifying your truck. A bucket of ice water and a usable pump are all you need. It may be a little challenging to find a pump with usable specs. I wouldn't be surprised if you decided the benefits aren't worth the effort.
Your chiller's pumps are probably too high a flow rate and not enough pressure capability. We always use a separate low flow and high(ish) pressure pump with a reservoir (bucket, cooler or tank) or heat exchanger. The chillers may also have trouble getting the water temperature low enough, or need some sort of anti-freeze at that low a temperature.
I wouldn't use any sort of flammable or toxic anti-freeze in the LCG water.
I would use some sort of bio-cide in the LCG water, it is really easy to get a mold or slime growing in the tubes, and almost impossible to get it out. We often used one of the brewing (or food service) sanitizers in the water. (note that the iodine ones will stain the tubes) A tiny bit of bleach will work, but doesn't last very long. Too much bleach may effect some of the plastics.
Hopefully this is some useful info, ask me any other specific questions.
