Thanks for the photos. I did notice that Nissan didn't run the floor rails all the way back to meet the rear frame box. I wonder why they chose not to connect them. It certainly creats a "soft spot" in the floor structure of the car. I guess they figured that the structure of the trans tunnel, the roof, and the rockers, would make up for it.
Based on my reading here, it looks like I will triangulate the front strut bar with the firewall, and try to connect the floor pan rails with the rear frame near the forward, rear control arm bushings. I will possibly also try to cross-tie the left and right floor frame rails near their ends.
I am doing this for suspension stability, and ultimately, more stable dynamic handling.