73 Duster after market fuel gauge with stock sending unit

Next step is to measure the resistance of your sender. If you don't care to crawl back under the car, you can access the wires at the "body connector" behind the driver's kick panel. Get a wiring diagram (search here) and multimeter (free w/ Harbor Freight coupon).

Oops. Well above is still good for the sender, but I noticed that you grounded the sender wire and gage only read 1/4 tank when should have gone past full. Your gage is likely OK and the problem is the 5 V regulator, the thing that makes the pulsing. That is an archaic thermal-mechanical device, the shiny rectangular can plugged in the rear of the dash. New ones are >$50, but better to get a new electronic replacement for $30. Search for "Dodge" or "Plymouth" "voltage limiter" on ebay (seller pink something) or see posts here for other vendors. BTW, your temp gage also uses that 5 V source. Does it read correct? Also, at last count a similar question has been asked >500 times recently on FABO so use search for more info.