I worked on the frame some today. Started cleaning and priming, but the wind decided to blow toward 6 plus figures of parked vehicles and I figured Momma wouldn’t like primer spots on her rigs... so I quit for the day. I did figure out how to use the antique door however... I think.
The red ‘4’ line is four feet from where I think the back edge of the floor will be. I’ll bolt a 6’ wide, 2x6 on the back frame member and use that as the rear floor support.