Frozen jugs of water near the cages are mandatory, in my opinion.
Heat is a killer, think child in closed car.
I would put them in last, at the back, I would so every hour or so and open the site to allow fresh air. All this along with something to keep it cooler, is frozen water jugs. Perhaps our the jugs in a Rubbermaid container and stack the crates on to of it. That will keep the humidity that forms from making everything else wet.
I would also offer bowls of cool water at the rest stops so they can cool internally and hydrate. Food shouldn't be an issue for six hours. I used to work in commercial poultry and six hours want uncommon, but the trucks were open so there was air flow.
A tractor trailer with open sides that sat over an hour with no air flow in the summer could experience noticeable losses. I'm talking a trailer of 9000 birds losing perhaps 100. Not huge, but noticeable. We would often spray a must of water over the trailer to reduce temperature. It helped a lot.
Monitor temperature! If they are panting, cool them down.
Or move at night when it is cooler.
