'68 Cuda Gas Tank Sending Unit

If the sender tube was 3/8.. the entire line coming to the fuel pump would also be 3/8. So, raise the hood and look at right frame rail. Unless you car was a big block build, I expect you'll find 5/16 tube and hose connecting to fuel pump.
And by the way... If you see the factories crimped clamp on that short rubber tube while looking there, you're seeing a rubber hose that is nearly 50 years old. It'll be a good idea to replace that with new.