If your long range plan is new tanks, don't waste your time on cleaning that old tank. You will probably knock some pin holes in it with the gravel or the acid (vinegar).
Whatever you do, don't try to run it on the old, unclean tank. Modern fuel is an awesome solvent for the old residue which has accumulated on the sides of the tank for many years, and that gum will end up clogging everything down stream of the tank, and gum up your valve guides and bend your push rods.
I'd bite the bullet and buy the new tanks now. Even though it would be fun to watch the carnival ride of the tank going round and round on the tractor tire.
Might as well hook up the planter to it and get your beans in so as not to waste fuel.
Carl
