kaosk-
My first reaction is one of dismay regarding the design of the bracket, but then many outfits build something then caution you against really using it least they be held responsible for it's safe use!!!
Most outboard manufactures and many boat manufactuers often caution against different towing position scenarios trying to eliminate the possibility that their component will be the one that fails.
In this case, what they're probably really cautioning you about is leaving the kicker on the mount/bracket when towing (especially over rough roads) where the kicker bounces forward and backward so much that it work hardens the metal in the bracket and the bolts, and gradually tears through the transom material as well.
The usual way to deal with this is to place a strap around the back underside of the motor and attach it somewhere forward of the motor and then tension it enough that the motor cannot flex back and forth on the bracket, stopping the destructive action. A ratcheting motorcycle strap would be perfect for this purpose.
You could also use a strap to hold it downward, but attachment points below the motor on the transom are harder to come by and more prone to problematic leaky behavior. Don't try to strap it down to the trailer, as the action between the boat and the trailer will cause additional strain.
Exactly how you accomplish the above will somewhat depend on the particular configuration of your bracket and motor to the boat.
Perhaps others will have other ideas on how to accomplish towing w/o removing the kicker. There are struts that fit between the trailer and the motor shaft, "transom savers", but they can (in my opinion) introduce more problems on a motor of this size than they will solve. Joe.