If you use any expanding foam to insulate the truck, be sure to use the minimally expanding kind, because it will push sheet metal out of shape if it is too concentrated in one area. Construction workers are well aware of this now, because of many mishaps when the product came out. The foam literally blew walls apart when too concentrated.
There are other products just now coming out that are a spray on foam application that both insulates specifically for heat and sound. I'd do some more research on what's available. I plan to do something similar for my truck, but I haven't researched it yet because I'm nowhere near that step in my buildup. I figured that by the time I am ready there will be even more products in this market.
