'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.

It was originally a /6....now a 318 - swapped by the P.O....