can you clean a fuel tank?

My son has started a new project, to make a long story short his beloved 73 Duster was destroyed in a crash when it was almost 95% done all the resto he wanted to do, so last week we picked up another Duster, this one is a 70 4spd car, not bad shape but we are virtually starting from scratch, it was a genuine "barn find" we loaded it up on a trailer after pulling it from the barn where it sat for the last 15 years, my son removed the fuel tank, the outside of the tank looks great, no rust at all, but the inside is a HUGE ball of rust the fuel we removed was vile! and I suspect had lots of water in it. I took the tank to a local rad shop where they said they could clean it in a costic tank but it would have to be lined, the cost...$250.00 the cost of a replacement tank up here in Canada where we pay twice as much for car parts as in the US is $275. and that's my shop rate discounted price, my son is trying to repair this with in the budget that the insurance company gave him for his car, ( this will include paint ) so short of a used tank, ( hard to find up here ) has anyone tried cleaning one with some sort of de-rusting agent in which case is there a inexpensive product to re-line it or dose he just bite the bullit and the budget and buy a new one?