[jonzcustomshop]

does your patch have enough material to go under the seam and be welded on where the factory spotted the piece on?

if it does I would cut your old one off about where the trim holes are, that will give you room to find the factory spot welds underneath, drill those out and remove the lower piece from the upper. you might be able to plug weld your patch back to the upper, or you could try that auto body glue that is used on newer cars, and just glue the patch to the upper, it will be difficult to get the seam perfect in the gap at the vertical piece- I suggest : wherever your vertical cut is- try to get the next 2 or 3 spots to the rear -on the piece that you are saving, then carefully wedge a chisel or putty knife in there to separate the rear piece just enough to get a cut off wheel disc in there (for grinding the weld)weld your patch to the rear piece from the inside, just do enough to get out of the seam, then grind and finish that little piece, before you weld or glue the patch to the front.

I did also see one truck where the guy drilled all the spots for the lower bed panel from inside the bed, he removed the lower bed panels and then he then cleaned the rust out of the crack and weld through primed both pieces and then welded it all back together.
a lot of work , but that might be another option .