I'm not sure if you have already gone and bought your parts, but have you checked to see if your gas gauge is the culprit? It's an easy and free way to make sure you aren't buying a sending unit for no reason.
Pull the little wire off the sending unit and stick another piece of wire inside of the connector and ground it to the chassis. Have someone look at the dash and see if the fuel gauge moves from E to F. If it does then it's your sending unit at fault. If it doesn't move correctly then you might just need a new gauge.
I just pulled an original sending unit out of a 67 Valiant. The sock was completely gone but the brass float and everything looked great. So pull the sending unit if you have to and see what it looks like. You can buy the float and/or sock separately to save money if the rest of it is in good shape.
