Alrighty, I've been looking at salt6's M151 restore...unfortunately he asked for comments on my experience rebuilding rusted out unibody vehicles....so here goes.
This ain't so bad. I've got a good friend who professionally restores classic sports cars. I've seen him rebuild a Jag E-type that was literally rusted in half!
First off -- unlike guns folks don't tend to store old vehicles by covering them with cosmoline. Don't I wish! Get rid of ALL of the rust. Grind, blast, wire brush, chemical, whatever you can do to make it go away. There's always more there than you think, and you can't weld to it. And it's like cancer; survivable but you've got to get rid of all of it. It's so much better to remove a little bit of "good metal" (that may only be half as thick as it was from the factory) than to keep it in and a year later see rust bubbling up from something you "fixed". The tin worm is an aggressive little beast, and it takes aggressive action to kill it. Some folks in the British car crowd use a product called Waxoyl that you spray inside the frame rails. It's exactly what the name implies, a waxy-oily product that creeps into all of the crevices and keeps moisture out.
The floors, battery box, and rust hole here and there in the fenders -- that's no big deal. What I'd want to do first is to clean those chassis members and get them welded up. Do you have a sheet metal brake? That'll be handy. You may be able to buy the original channels, but being able to bend your own will help in situations like http://www.panzer46.net/m151a1/DSC02220.JPG
shows. You can make a U shaped bit that will fit inside that rusted rail, weld it in, and go from there. While this kind of construction is called "unibody", it still has a frame. It's just that the frame is welded to and gains a lot of strength from the body panels.
Check out a company called Eastwood. Many of the goods they offer are overpriced and available elsewhere, but they do carry a number of high quality top of the line supplies you may not be able to find at the local car parts place. Plus you can get an idea of some nifty little restoration tools and tricks. The seam sealers they carry are top notch. I seam seal all of my underside and exposed welds. You may have noticed a weakness of this type of construction, it looks like the mutt was originally spot welded. Rot will set in between the spot welds where the panels overlap. I've removed a number of floors from vehicles like this with nothing more than a chisel and a sledge -- the spot weld area was that rusted. I prefer to seam weld the floors in, and then use the paintable seam sealer. After you get the seam sealer on, dip your finger in some solvent and you can make a really nice smoothed surface without making a mess of everything.
Pick up a set of body hammers and dollies. Even the best replacement panels you buy won't fit right -- they probably weren't made on the original tooling, and if they were that's some _old_ tooling, plus your vehicle no longer has its original dimensions. Fortunately you don't have to worry about a big concern we have with little convertible Brit cars -- getting the doors to work right again!
What are your plans for finishing? I swear by self-etching primer when I've got something rusty that I've stripped down to bare metal.
Keep us updated....driving a car you've (re)built with your own hands may even be more fun than shooting a gun you've built...and in your case you've got something you can mount the gun on!