I think mounting and unmounting the motor over and over is going to get old very quickly. You can really hurt yourself with those motors mainly because of the awkward geometry, and most boat transoms don't really like to have those screw clamps dragged over the wooden support block too many times. You should spend your time figuring out how to secure the motor on the boat, and not how to secure the motor on the truck. There are many devices out there which have already solved your banging motor problem. The one I like best is the lower end "crutch" with a ratchet strap holding the lower unit against it. The bottom of the crutch could have a special "pocket" to slide into welded to the trailer. While you're at it, weld a couple more hooks to the back of the trailer so you can fasten the transom of the boat to the trailer without having to strap across the gunnels. For motor security, there are plenty of locking solutions out there, usually involving a C channel type unit which slides and locks over the clamp knobs.
Spence