Hello from Cave Creek AZ

Desert Classic Parts said
I can rebuild original units. Changing the tube size is a really big job. I can do it. A standard rebuild is $90 including the shipping in the USA. Changing the tube would run significantly more. I've done a few where I added a return line for EFI and a tube refit would actually be much harder than that.

You do have a point. Let me think about this and see what I can figure out

You'd probably have to invest in tooling, fixturing, and/or sub-part manufacturing of the 3/8" to reduce your labor $$ cost. $90 for a good part over a $60 semi-functioning part is palatable. You start going over 3x the lesser fuel senders, customers start looking at other options or just put up with lesser fuel sender.