What width is the rim? Many stock rims are not wide enough for 265's, much less 285's!
With that said, their are some cases where having a slightly narrow rim is better than wider, like off roading, this protects the rim, along with potentially getting a bead removed at lower psi........ Reality, if loading heavy, you need the correct width rim. I noticed lack of handling with the stock 6.5" rim, vs the aftermarket 7.5" rim with 265's, on my 88 and 96 SW 3500's. I can not in all contagiousness recommend 265's, much less 285's on a stock 6.5" rim, if that is what you have.
Marty