Finished welding the perimeter from below. Replaced the body mount bushings on the pass side, started in on the rear seat area.

Welded the pan from the underside, but didn't take any finished pics. This is after I'd started tack welding every couple of inches. I have realized that welding upside down is painful when weld spatter lands on your arm.
I installed the new body mount bushings on the passenger side, except for the rear brace.. it was too damaged. Here I've created a new plate out of 16 ga steel. 16ga steel is very hard to cut with tin snips, but should last a while.
After looking at what came out, it looks like these nuts were just welded to the brace. Tomorrow, I replace sheet metal in the rear seat area.