I've had heat issues as well, but perhaps on a smaller scale.

When I first got my shop truck, it had a superwhamadyne high performance carburetor installed, and the fuel line tested the effects of gravity as the line ran over top of the air compressor.....





I replaced the carburetor with a 2 bbl Holley, and ran the fuel line under the air compressor bracket, installing a fuel filter on the fender well.











This worked fairly well for the limited putzin around I do with it, normally only a few miles at a time. Then came last Tuesday and the forcast of 100+ degrees weather. Got in my daily driver to head to work, and the PSD's starter had given up the ghost after 170,000 miles. Can't complain too loud about that. Still needed to make it to work, so I fired up the beast and it made the 20 minute drive without issue. ........until the return trip that afternoon. Temps were around 105 degrees, cowl vent barely helped, and the first substantial hill came after about 5 minutes of driving, so it was good and warmed up. I barely made it up the hill, the engine was spitting and snotting, and luckily for the impatient drivers behind me, there was a paved shoulder to the road. I finally inched to the top, found a safe place to pull over, and surveyed beneath the hood. I felt the fuel filter, which at this point, and considering the proximity to the exhaust manifold, was too warm to keep your hand on it. The worst hill behind me, I did manage to make it home, using the paved shoulder for most of the trip. Changed out the starter on the diesel dually that night, and decided to address my fuel line on the F7 the following night. Removed the fuel filter from the heated area, and installed a steel line in place of the rubber hose....














This still left the line in close proximity to the infamous Y block crossover pipe. To help protect it from as much heat as possible, I made a stop by my buddy's motorcycle shop for some used HD (sportster, I think) heat shields........








....and after many fittings and trimmings.....hey, this is the shiniest thing on this truck...











....which helped out considerably. On your truck, the muffler is a major heat source but also a heat sink. There is quite a bit of metal there to hold the heat, so naturally it will dissipate for some time afterward. I don't know that a heat shield would be the optimal solution, I think my first inkling would be to get the mufflers out from under the cab to move that heat source elsewhere. If that is not a feasible solution for you, then perhaps a shield over top of the muffler would help to keep some of the heat off the bottom of the cab.. A racing buddy uses asbestos wrap on headers that may be another solution around the muffler..