Metal cleaned up pretty well with the sand blaster. There was some sort of undercoating that wouldn't come off in some areas, I'll have to take it off with the grinder.
Here is the bad area on the driver side.
I put a quick coat of rustoleum metal primer over everything I blasted to keep it from flash rusting. I figure anything that isn't perfect can be re-blasted and repainted if it shows any discoloration after a week or two. Also, the areas of the frame that get repaired will have to be repainted afterwards as well.
This is the area on the passenger side that is rotted out. The bracket looks to be in good shape, but what it's attached to is definitely not.
I figured out why it was rusting all around this area. The frame was totally clogged with mud, about 4 inches deep inside there. The mud in the frame probably retains moisture and causes the rust from the inside out.
I cut that section of the frame out with the cutting torch and all that mud fell out.
I was able to drill out the plug welds all over the bracket and remove the rusted part of the frame from the bracket. They don't make replacement brackets, so I needed to save them.
I'll sand blast this inside and out and repaint it before welding it back on.
I got a bunch of small pieces of steel for free from the local steel place. Here I'm mocking up the plate on the drivers side to put on the passenger side. My plan is to cut it to follow the original curve of the frame on the bottom of the tape line, then make another one and weld them on the inside of the frame on the passenger side. Then I'll go back and plate the bottom with some thicker steel.