Ok, just commenting on this in perspective. I do motorcycle tanks, so obviously less expensive.
I use Rusteco ( expensive allmost $300 for a 5 gallon bucket )
I use that to dissolve and treat the rust, it also pretty much cleans it too, and is non toxic.
The pebble ( or what have you ) method works for loose scale.
I made a simple tank tumbler up for the cycle tanks, and use ball bearings, nuts, bolts, rocks, even broken up old car windshield glass, it all works.But in your case you would have to shake it around by hand, so I wouldn't put a whole lot of liquid in it , at 8+ lbs. a gallon.After that -
I rinse and rinse and rinse and then get it dry , and the NAPA sells the POR brand stuff to coat it with.
Coat it ASAP , bare metal will oxidize ( though not seen by the naked eye ) in a very short time.
Car exhaust makes condensation inside the tank.......
I have seen places listed that will do this,but it may be location specific,I am in NY.
Good luck .