I don't think the transmission mount is a good idea at all, it allows for much too much movement. Plus, there is no way I would use it unless it is innerlocking. There is a reason those type mounts aren't used for mounting bodies at the factory. First, I replace broken motor mounts all the time from collisions. The inertia of the motors weight continuing to move forward in a collision will tear these mounts RIGHT NOW. Yes there may be a number of them but when one breaks, the chain is likely to continue breaking them. Second, they move so much. Especially with the bed welded to the body, you are going to need that unit to be mounted pretty solid or there will be cracking at the joint of the two. Plus, if the body moves too much there will be all kinds of problems with brake lines, cables, steering linkage, etc. Everything that runs between the body and frame is likely to have extra stresses on them that don't exist with a normal body mount.
This isn't an issue if it is only used on the bed but in this rig there is no "only the bed" because the two have been welded together. You can't have one half of the body able to move and the other half not, you WILL end up with cracks and failures.
Yes, you could use them at certain points, but don't push it and use them at too many places.