Time for another update. Did some serious reinforcing around the back of the cab / front of the bed. All I can say is WOW, what a difference from one side to the other. Guess with all the interior and roof, I never noticed how flimsey the back corner of the cab was. Right now, you shut the drivers door hard and the cab corner and bed side shakes some and sounds weak. The passenger side, on the other hand is probably about as stiff as the daughters truck - absolutely amazing.
After tearout
1/8" x 3" plate welded across the bottom of the opening. Cab sheet metal folded in and welded. Note additional filler piece sitting in the bed.
Cab portion done. Have not tied into the drivers side yet, but cab corner is now really stiff.
Angle iron filler added to the front of the bed (vertical & horizontal). When the bed is reinstalled later, the gap will be filled with silicone, then upholsterd.
Also trimmed up the drivers side door at the mirror. Cut line and gasket sealed with silicone and will create new interior cover to fill in the former window track opening.