This is my back half idea.
red planes: Body/ location for old/new frame meet
blue planes: Angle for the tube to be cut
Purple planes: heights of the frame
black planes: body mount
The other little bits sticking out are fish plates and yes there will be plates on the inside aswell.
Made with 2.5x4" Rec. tube .188" wall. And i will make the bumper angled and D-ring mounts will be in there somewhere