Chris
I'm gonna have to steal some of your technique for my brother's bus, it came from colorado with holes in the floor, wheelwells, heater system and a hole about the size of a brick in the bottom of the left rear quarter. Still not finding much rust hiding in my Pinto, I've been under the carpet, in the trunk, carefully examined the firewall and cowl. I did find a few small bubbles on the edge of my trunklid though, and I'll get a pic of my 'rusty' battery tray up for you.
Chuck