How would you run the return ??

"The sender has the small vapor 1/4” fitting so I’m not sure I want to drill that out and try brazing a 3/8” tube on it"

First the electrical portion of the sender is at least 3" away from the heat... So nothing to be concerned with..

Second I agree let a radiator shop do it, they deal with this kinda stuff all the time... But they won't braze it, they will solder it.... Less heat....