Replace gas tank and sending unit or not?

Hey everyone, sorry to drag this thread out of the shadows but I am having a related problem.
74 Dodge Dart Swinger 318 that is only taking 4-5 gallons. The pump shuts off and we can't fill anymore. My son normally drives it and the other day he ran out of gas (teens). He was only able to put in 5 gallons. What should I be looking for here?