How would you run the return ??

rmchrgr said
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.

It was just a suggestion and another option. "What exactly?" Nothing exactly. There are lots of things you can use for fittings, JIC etc steel, various brass, or copper or steel tubing. Frankly, I would find it a lot easier to punch a 3/8 hole and braze in a piece of tube than drill a 9/16 hole. IT IS ONLY a suggestion.