I bought a 3/8 sending unit. Wrong Unit?

I have a 5/16 unit i pulled and bought a 3/8 fuel sending unit for my 63 dart. I have a slant 225 though. Can i still use my original fuel line? Should i send my new unit back and get a 5/16 with out a return line? My old unit does not have a return line. If i can still use the new unit i will. I want to. Please give me some info. I saw the link on here for 40.00 with shippingand couldnt turn that deal down.