Boy, and to think a few yrs ago, I had a sw3500 with a whopping 12K gcwr! I used to run down the road at 20K, no white knuckles as one person was saying I would have had....oh wait, it WAS a deezal twuck!
Until I personally see WHAT payload, frame axel......pkg this thing has, I would not doubt a 400/400+ motor should not have issues with a 12K trailer. I routinly pull a 12K trailer with a 12K empty twuck and a non turbo IDI deexal with a 175/330 power spec motor. Pulled said trailer with my 12K gcwr twuck and its lowly 6.5td deezal too! no issues, other than being slower than an equal setup chassis rig due to less HP and torque!
Marty