To be honest here, I feel cutting a tank apart to clean it makes for a lot more work.
If the tank is sound enough, but has some rust on the inside, there's a whole host of chemicals available these days that can be used to clean them up, or another popular method is if you can get a hold of a cement mixer, wire the tank to that, so you can tumble the tank, fit an old sender, put 2 handful of 5 and 10 mm gravel in it, an old cap on, and let it run for about 2 - 3 minutes, have a look inside to see how it's cleaning up, usually within 10 minutes it'll be done.
I'd also suggest removing the gauze filter that's in the tank. If you want a 'filter' here, fit an external one, at least then it'll be serviceable.
Then clean it out, fresh water will be OK, use a POR15 Kit and follow the instructions to the letter, even here, the cement mixer is handy as you need to tumble the tank over and over and over,,,, The tanks I do are usually new and clean of rust, so I use an engine stand to tumble the tanks for the POR15 process. One thing I'll add here is don't under estimate the time this will all take.
The Tank halves are resistance welded together originally. Any rust that resides in the seem here, the preparation chemicals in these fuel tank repair kits will deal with that. I find with the POR15 Kits, they have available one that's size to do motorcycle fuel tanks, that's fine to do one Mini (or Moke) fuel tank. If you follow the instructions, you'll get 20+ years from the tank, as long as the outside is in good order.
Don't forget to blow out the breather and fuel take off pipes as the last part of this process !