The sizes I referred to (1/4" and 5/16") refer to the size of the line, not of the threaded fitting.
Maybe the best thing would be to take your valve to an auto parts store and check to see what size brake line fitting screws in. These have flare fittings just like fuel line. Then you would know what valve to buy based on the size of the existing line. Be careful when you buy the valve to match the size unless you want to put in a new line or reducer.
Both styles of valve will bolt into the tank interchangeably.