When you buy your Dinitrol get something to apply it in to cavities with. You can buy several cans which come with extension pipes with multi-direction spray nozzles on the end, they work well. As well as the chassis inside and out,including the outriggers, other places to focus on:
Up inside the door pillars, all the way up and around the corner at the top and along under the windscreen
Seat box ends from under the vehicle.
Where the chassis outriggers bolt to the tub. (seat belt mounting points through to chassis)
Where the door post support brackets (Just under the seat belt reals) bolt through the tub.
Around the fuel filler pipe tunnel.
Rear wheel arches, low down towards the rear of the vehicle.
Under the rear wheel arches where the steel plates bolt through to the seat belt fixing tubes inside. (CSW)
Through the floor joists which the rear floor sits on and is riveted to.
Inside the door frames.
Everywhere underneath needs treatment really, but places where steel fixes to or touches the aluminium are far more important. How often you treat it depends on how you are using the vehicle.
I've probably missed other important bits, but some one will add to this list, I'm sure
