I think I have heard of radiator shops also flushing gas tanks. Maybe you could try a couple local calls to see if they have experience doing that.
Another possibility might be chrome shops, as they have to soak parts in various acids and such for plating. Maybe they would also know the common proctices to recondition the metal tank.
BTW... I thought the TD was a great looking car! They showed up sometimes in NHRA Modified Sports classes in the '60s.