I've been sandblasting, actually using BlackBeauty....sand is cheap or free but the Beauty won't kill ya. If blasting get a hood, they are cheap, around $25. After the blasting I have been coating everything with Zero Rust (couple of coats).
I'm not using undercoating, had terrible results with it in the past.OEM works great but doing it aftermarket just doesn't seem to do as good of a job.
I have been using seam sealer, seal every 'lil space you can find and bedliner. I'm using two coats on the pans and any other sheet metal I find and one coat on the "frame" rails.
I removed my fuel tank, it will be one of the last things to go back in. I left the fuel lines attached to the rails, however I am replacing the brake lines while I have easy access to them, along with the exhaust, removed and blasted the diff. and replaced the rear brake's and ALL RR brake hardware.
If you can see the floor rotted from underneath then be prepared for a lot of work,the best thing to do is to pull back the "cancer causing" carpet and take a look.
Good luck with yours, I couldn't imagine having to do pan replacement for a living, I'd have to take "Anger Management" classes every week...lol.