First of all, congrats on your new-to-you Bronco! The '72's were a good year in many areas.
Well, for the auxiliary (side) tank you won't have any rust issues other than some debris from an extremely rusty sending unit. But that's not as common as other rusty areas on Broncos unless you're in a moister climate. Hmm, GA eh? No moisture issues there I'm sure!
Could explain the crusty filler necks.
A simple cleaning of the plastic tank then would be pretty straightforward I would think.
For the rear main tank, other than the plastic '77's rust could be harder to fight. Like was already said, if it's light you can clean it and move on with your life. If not, then I would spend the money (I know, easy for me to say...%)) on a new and probably larger tank.
As said too, you can get a stock replacement pretty inexpensively. If you're going to drive this Bronco much though, you would be well advised to at least consider a larger 23 gallon replacement.
If you're going with EFI in the future (especially if in the near future) then it's good to just get one already made up with the in-tank pump ready to run.
Or, a newer option is to get the standard tank and add one of these:
http://www.wildhorses4x4.com/product/FiTech-In-Tank-Retrofit-Fuel-Pump-Kit/newproducts-EB to it.
It would be slightly less expensive, and looks to be pretty slick setup.
Never used one myself though, so can't speak from experience.
The EFI ready 23 gallon tanks are more expensive by a bit, but you do gain the added capacity and the ability to work with a body lift if you get certain types.
Not that the last bit makes much difference when comparing to stock though. The only real advantages would be the added capacity and the better baffling for fuel control.
As I said, it's easy for me to spend your money for you. I'm a vendor too, but think I'm speaking pretty logically in this case. You can still choose from several of us Bronco vendors that have good products.
Shipping even becomes an issue with something this large too, so that's another aspect that's at least worth adding to the mix of things to consider.
Good luck!
By the way... Is the 5th digit in your VIN a "P" or a "Q" by any chance? If so add your info to Viperwolf1's thread about that. Keeping track of the changes between the '72 and '73 model years, and exactly when they were implemented.
It's a fun thread anyway though, so check it out.
Don't have a link, but if you use the search function above and go to "advanced search" and search for threads STARTED by Viperwolf1 it should pop up near the top of the list.
Paul