This is what I came up with for the replacement of the inside step.
It's 16 gauge steel and I'm still planning on bracing the back side with something. I haven't decided what yet, probably angle iron.
I had to use two pieces and i flanged the second one to make it fit near flush. It looks nice! Much better than the hole that was there. I'll do the welding on the other side after i flip the cab back up in a few days hopefully.