[dsraven]

good idea to clean off both sides of the panel when welding. the puddle can draw contaminants from the back side otherwise.
if possible to fab up some heat shields I would def do that. some light aluminum is all thats needed really. since the truck is fairly low it will hold heat under there especially when stopped at a light etc. stuff like rubber park brake cables, rubber or braided brake flex hoses, fuel lines etc. even the brake lines as they will heat up the brake fluid and cause premature breakdown (one of the reasons they say to flush the brake fluid every year). maybe for the part of the pipe that is close to the bed wood you could use a length of oval pipe to allow a heat shield under the wood. if you had time you may find a preformed heat shield that would work at pick n pull.something from above a resonator maybe as they are usually smaller than a catalytic converter or muffler.
anyway, it is your truck so you decide if you want a heat shield or not, just giving my opinion. as a proffessional firefighter (retired) I have been to lots of vehicle fires, never good. as a lic'd mechanic I have seen melted/charred carpet above missing heat shields.