I found a couple of spots of rust bubbling up on the floorboards. This is ahead of the front driver's seat where the floorboards make a 45 degree turn to join the firewall, right where the "frame" joins the floorboards. Turns out the part of the "frame" that flanges to the floorboards was rusty as well as that part of the floorboards.
Here's the area after a new "frame" flange was welded into place:
Here's the patch panel for the floorboards:
And here it is after it was welded in:
The same area on the passenger side was rusty as well:
Here it is after the new "frame" flange was welded into place:
Here's the passenger side floorboard patch:
And here it is with the repair competed:
With a little luck this completes the rust repairs on my SX/4 - WooHoo! There's some of what appears to be minor surface rust around the windshield so I have to remove that and sandblast it before I'll know for sure if all the rust repairs are completed, but it looks like it at this point.