Use a good oil based undercoating on that 7.3. Use a thick fogging oil like "eagle one" and spray over the rear fender well 1 can each fender) from under the truck, you have to soak that foam with oil or your bed will rust out; once it's soaked follow up with a layer of oil-based undercoating of your choice such as fluid film etc. (The amount of undercoating needed to keep that truck in like new condition is measured in gallons btw.)
I don't want to see a 7.3 powered truck become a casualty to rust (and I'm sure you don't either). The 7.3 is, imho, one of the best engines ever made. Good luck.
Make sure you use only motorcraft, racor, donaldson OR (if the oem fuel filter cap isn't there) use baldwin or luberfiner fuel filters only. I should have known better and I almost cost myself a brand new set of injectors. Additionally, those are the only brands of fuel filters that meed or exceed the micron and water stopping efficiency required, many other aftermarket filters (napa, wix etc.) do not even come close. Failed MANN fuel filter- 7.3 powerstroke
any questions about (what I assume is new to you) your 7.3 powered truck feel free to PM me.