What exactly are we brazing onto the tank though? I'm not sure it makes sense to just stick a fitting or some such thing onto the outside of it. The bulkhead fitting I linked to was literally made for this exact purpose and from my point of view it looks to be much easier to deal with and a better solution.
Understand the OP has a new 3/8" sender but another option could be to get a different one with a built in return nipple.
Like this.
