5th pic with the bracket mount installed I used masking tape again to gauge where the best mounting area was to get maximum edge margin for the Pioneer 6x9's. Moved the speaker up to full travel within the cut out, marked it, then down to full travel within the cut out, then side to side for same. Split the differance with the speaker by marking the center of the travel ine limits. Marked the speaker mount holes with a pencil, used a spring loaded center punch to mark with so drill bit would not walk on me.
Next pic shows the speaker mounted using # 8 x 1" drill screws. Drilled the holes undersized and then screwed the fasteners in so to "cut" threads as it goes in.
Last pic shows the grill installed with the bracket competed. Still need to remove, de-burr, clean, prime and paint the brackets but that's another day. Now need to find best way to mount my Polk 5 1/4" speakers under the dash as don't want to cut door holes or use kick panels. Just personal choice on my part.
Most likely not the best write-up on this but then again, I'm not a tech manual writer either.
Should give the general idea of how I did it. All in all very satisfied with Jeff's solutions for corner speaker mounts. Again, WILL NOT work with in-cab tank still installed.