We have been selling Classic Mustang parts for over 25 years now. During that time, there has never been a Gas Tank/Fuel Sending Unit available with a 5/16 inch inlet. All of the 65-early 67 Mustang originally had a 5/16 inch fuel line. The service replacement part from Ford has had a 3/8 inch inlet tube for over 30 years. It was not really a big deal, because a 5/16 inch hose could be made to work.
It is just nice that the correct one is now available. Not only does it have the correct inlet tube size, it is stainless steel construction along with the original style brass float. All units are tested by the manufacturer before being shipped.
We also have cheaper "standard" sending units. Give us a call or check our web site.