Jr.; There are several products on the market to clean and seal fuel tanks. I used U.S. Standard Fuel tank sealer and Metal Ready from Por 15. The Metal Ready will clean the tank as I believe it is a phosphoric acid. Once the tank is clean and dry you pour in a quart of the sealer and slosh it around and then let is drain. It applies a relatively thick coating of sealer on the inside of the tank. For more info on the product you can check out "Por 15" on the internet.
There are some radiator shops that will seal the tank for you. To do it myself it was a relatively easy process, just time consuming.
As far as worrying about it, it really depends on how bad the rust is. One thing you don't want to happen is a fire due to gas leaking on a hot manifold, etc. My tank didn't leak but like you I found rust inside and in some spots the metal appeared thinner than others. To be on the safe side I sealed the tank. Hope this helps.