It's the long brake line running from front to rear. You'll need to replace all the brake lines anyway, so buy a brake line kit from J.C. Whitney. It has everything you need for the entire car, plus the long line is split in two parts, which makes it easier to install.

You'll also need all four wheel cylinders, a master cylinder and the four rubber brake line hoses. You can source those parts from whoever you want, but I recommend the Whitney brake line kit because it's one-stop shopping, and a lot cheaper than buying them individually.

If you set out to rebuild a car in this condition you must replace all these brake parts.