My friend had the same problem.....
A 53 1/2 ton with a V8...It was awful. Turned out he was getting a load of heat sink from the exhausts.
He spent a bit more time (and money) on good insulation on the whole floor including under the seat and the firewall.
Making sure there were no openings or gaps.
It made a big difference,mind you we don't have to deal with the same kind of ambient air temps you guys are dealing with.
It could be you're getting quite a bit of radiated heat through the roof as well.
