From what I gather the 68RFE is a very strong transmission, and the Aisin is near unbreakable.. you also gain 15/65 in terms of power.
That being said I would be less concerned with GVWR and really only care about RAWR and FAWR, don't exceed those and you will be fine. Same frame design, different suspension, as the 14K GVWR 3500 trucks. The suspension is designed for RAWR.. so run it up to that. This isn't a 1/2 ton at the top end of it's design spec, it's a 3/4 that is always well below design spec even at RAWR. This will give you more than enough carrying capacity for the 5er you mentioned, as there is about 3,400lbs to spare in RAWR from empty.
I also would get the prep package.