Unless you are restoring your truck now, as in cutting and replacing sheetmetal, and you want to stop the rust that's on parts you don't want to replace you are wasting your time.
In the 50+ years your truck has existed it has only rusted as much as you see. It's not going to get that much worse in another couple years until you get ready to cut and replace parts. In fact, the rate at which metal rusts decreases over time because the rust that's there actually slows the process by blocking some air from reaching the virgin steel.
Save your money and concentrate on first things first.
