Yep, the bigger line is the supply. The fuel pump is 3/8" pipe thread, not sure if they gave you the wrong stuff or what, but I'm 99% positive that the pump is 3/8" threads. Is it in yet? If not, take her with you to the parts store.
P.S. DO NOT go mechanical pump at this point. PIA to install, you'll need a new front cover, and the pump drive eccentric for the camshaft, and depending on what cam is in the car, it might not even have the correct holes in it to install the fuel pump eccentric. Then there's the whole rerouting the fuel lines, buying more fittings, making a steel line up to the carb, etc etc etc YUCKY!