Remember- the ratings are based on continuous use.
IMO- 400 pounds is no big deal. Other than keeping good tries on the truck and religiously checking the tire pressure before each haul (which should always be done anyhow)I would not give it a second thought- just run it.
You might want some airbags which will do nothing to the ratings of your truck but may enhance ride comfort when loaded-
For comparisons sake- My last truck logged tens of thousands of miles while 2k over the pin weight and 8k over the gcwr. Got 38k for that truck when I traded it in-
That truck ran just as good when I traded in as it did when I bought it.