Replacing fuel tank on 1966 Barracuda, which sending unit?

It'll depend on the what you have for engine. If you're just running a street car with a mild engine, the stock 5/16 line is just fine. If you have a megahorsepower monster under the hood, you might need to go bigger.....but you'll have to go bigger all the way to the carb or it won't do much good.