Although I haul Deere combines, they are all about the same. I load combines on the trailer backwards for several reasons. First of all, the windshield on the combine faces backwards, so I don't have to worry about a rock or something else taking out a $800+ windshield.
Secondly, on a combine WITHOUT a head and it's engine in back, the rear of the combine is heavier than the front. I want more weight on my tractor half of the trailer than the trailer half. A combine is also slightly more aerodynamic when transported backwards, and every little bit helps.
Most of my customers have duals on their machines with 30" row spacing. The combines that Johnson Harvesting use have the fat single tires, which end up being about 13'-14' wide. With duals on 30" spacing the center of the outside duals are 15' apart, which makes the overall width around 17' wide, as noted above. With the duals toward the rear of the trailer, I am much more able to see both outside wheels without moving my mirrors. This also allows me to better see vehicles following me.
I will not haul a combine that has duals on 36" row spacing--the duals are 18' wide at wheel center, yielding 20' overall width. I pull a Palamino(formerly Wilson) MUV-ALL and with the sides fully extended the trailer is 13' wide. The inside duals are 12' apart on center so that means the only the inside 6" of the inside dual would be on the outside rail of my slide-outs. NO WAY IS THIS SAFE AND I WON'T DO IT!!!! Luckily most farmers are phasing out 36" row spacings, so this is becoming less and less of an issue.