The best way to keep heat out of your cab would be to use radiant barrier foil but for it to be very effective you must install so there is a 1 inch gap between the foil and the surface you don't want to get hot. Around 12 years ago we installed that stuff in our attic on the bottom side of the joists & rafters of the roof boards creating an air space between the foil and roof sheathing and the use of electricity for A/C dropped by over 28%. Some of the best money I ever spent. If you want to see just how effective this is, just get a 60 watt incandescent light bulb, turn it on and hold your hand about 2 inches from it. You'll feel heat and it will increase the longer you don't move your hand. Now do the same thing but with a piece of common foil (shiny side facing the bulb) between your hand and the light bulb. You will not feel any heat at all.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end