Floor replacement: what went under your plywood?

Once the seats are out and the old floor is up, everyone makes a different call.

• Rust treatment: what did you use, and how's it holding up?
• Insulation under the floor: foam board, spray, nothing?
• Subfloor: 1/2 or 3/4, screwed to what?

Photos of the layers before they got covered are worth a lot here.