The 6.4 w/3.73 will pull that trailer like a dream. Just traded one off. Off hand I think you will still have the 6 speed tranny but that trailer won't strain you at all....and, the stability will be SO much better. 3.55 w/5.7? I had that. The 6.4 combo will be far better. With that combo you didn't need, or want, the 4.10. Just bought a 19 3500 w/6.4, 4.10 and 8 speed. It sounds like my old hot rod with a 4 speed close ratio and 4.10? (4.56?). The shifts just keep hitting...thankfully there are 8 or I might run out about 65 mph.