[dsraven]

its a good idea to mock up the wiring and hanger points etc but then remove that, install the firewall cover, then install the wiring so it is exposed. wrap the harness in whatever you choose then hide it with whatever you choose. modern cars use kick panel covers etc. I was a proffessional firefighter, before i retired a couple of years ago, and have been to lots of car fires where the home grown "big stereo" groups would route wiring under the carpet and back to the truck. sometimes wiring would chaff on an edge or a component would short out. long story short the crwes responding had no access to the problem and the car would burn up. had the "group" wired the vehcile with proper fuses and left the wiring exposed the driver would have smelled something hot before it started fire and possible been able to disconnect things before it was too late. before I was a firefighter I was a heavy duty mechanic. back in the day some manufacturers ran wiring with all the same color and used to define the circuits with numbers labelled on the wire every few feet. yeah, fun trying to run down a problem for sure. the same would go for wiring hidden behind carper or a firewall pad.
it's your truck and you should build it like you want it. it's also probably you who gets to work on it after it's done. just saying.