blacklung

I have a 1976 Chevy K5 blazer. Found out the previous owner swapped the 400 sbc for a 305. When I dropped the fuel tank to clean it I noticed that the tank of course has the filler and vent hose and the sending unit has the feed and return line. Well their is anther line next to the feed and return that has about 4 inches of hose and is cut off. My question is what and the hell could that line be for? When the vehicle is running no fuel comes out of it. My next question is could this line being open allow my carb to be loosing its prime after a day or so and be the reason I have to re-prime it and then it picks up fuel? Could I just plug this line off? Any suggestions or help would be great!