Best method I found on the internet was to fill the tank with something that will break the rust apart (sand,rocks,bb's,nuts and bolts), then cover both fill and petcock hole with lots of tape so nothing will come out. Then wrap the heck out of the tank in thick blankets and such and tape them so they wont come undone (just acts as a cushion around the tank). Then put it in the dryer on the setting with no heat for an hour or so and let your homemade sandblaster go to work!--just dont let the wife catch you
Then pull it out, unwrap the blankets and open up the fill hole and dump out the media you filled the tank with. Out will come all the rust and the inside of the tank will be left clean metal.
I used this method and then I used Bill Hirsch's products for cleaning and coating the inside of rusty tanks and mine to this day (over a year later) looks brand new and works like a charm!!
P.S. The stock petcock will probably be useless since the screen will be caked and covered in rust particles. If you are doing this on a Savage I would recommend converting the petcock to the Yamaha Raptor petcock and never look back.
HTH
-Matt