I'm tire of running out of gas in my 84 chevy pickup

[JAYBEE]

I ran out of gas yesterday for the 2nd time in my 84 chevy scottsdale. I replaced the sending unit in the tank with a new one. I replaced the gauge with one from a salvage yard. Before I did anything to the truck the gauge would sit on E, until it would get empty, and then the needle would bounce. Now it just sits on E. I need Help. How do I test the gauge? How do i test the sending unit. I am tired of Walking. Keep in mind the truck has had many changes over the years, but the electrical under the dash is still stock.

Thank you for any help