The planks on my trailer are under a piece of 2x steel on the back but only a 1 inch piece on the front. If cut to the right length, new wood can be slid under the rear, dropped into place and slid forward until it stops. This leaves each end held by one inch. When I got it, one screw held each plank from moving around. When I redecked, I never put any screws in. Now one has slid back enough to get out from under the front hold down and I can't get the &^%* back in place.
My dad's truck(s) had a 12 foot rack. It was fairly standard procedure to haul 2 tractors (larger than Cub size). Tricycle types were pretty easy. If they had wide fronts, they had to be overlapping like George suggested. You just had to load/unload them together.
As far as I know, the legal width on all interstates is 102". State highways in most, but not all, states is the same. Others are 96". Length of a Cub is between those 2 figures. The trailer does not have to be as wide as the load, as long as overall is inside the limits. This doesn't mean you won't run into some moron with a badge. Also on a narrower deck, you are probably gong to end up with more overhang on whichever side the rear of the tractor is on. Since the rear is already the heavy end, you will end up with the weight off center. Being heavy on the left is preferable to the right because of the crown of the road.