I'm in the midst of converting my '83 over to fuel injection. I am using a '93 tank. I took the tank to a local radiator shop. For $50 they pressure tested the tank to make sure there are no leaks, then they flushed it. They also treated it to prevent corrosion. They called it "seasoning". I have no idea what product or process they use.
In my opinion you should just take your finished tank in to a radiator shop and have them go through it for you.