It looks like you've already cut back the original floor to at least the edge of the seat riser flange. If that's the case, I'd cut it back a little more to the middle of the flange and butt the new floor up to it and weld the new to the old using the riser flange as a backer for the weld. I just did my passenger side and left the battery compartment in place. I had enough original floor to be able to butt weld just forward of the seat riser toward the tranny opening. I also left the rear portion of the tranny opening in place, butt welding at the corner of the opening.
Here's a pic of mine from the inside.
[on edit] It looks like your cab is still on the frame, so welding from the bottom probably won't work. And I didn't notice originally that you have the new panel laid on top of the existing floor.
You might consider leaving the battery box in place like I did if it's solid enough so that you won't need to weld from underneath.
BTW, please clean out the junk from inside the seat riser. You don't want to burn it down.
