I'm trying to trouble shoot my gas gauge problem. The truck (91 K5) runs out of gas when the gauge reads 3/8 full. Would this be the sending unit? Also, I have done some research and all I see as the tank on my year is the 31 gallon. They didn't offer the 25 gallon that year? Lastly, if/when I change the sending unit, is it a good idea to change the fuel pump? Best to go with a GM unit? Thanks in advance.