gas tank cap

[MikeyMan]

Gas caps can be found at most auto parts stores. Look them up online, and buy in-store. You may want to take old cap for comparison, sometimes these tanks get swapped and may have a different size filler neck.

Your 72 may also have some emission related extra hoses. Some have evaporation line that runs through cab then frame rail to canister in engine bay. Though most of these systems have been bypassed/modified/eliminated, there's a chance a deteriorated line could be the source of fumes in cab. I've also seen a short rubber "vent" hose near the filler neck inside the cab. Check and replace ALL rubber that is inside your cab.
My first vehicle was a 69 C-10, and I was getting gassed out. Turns out the tank itself was rusted through causing the fumes. Had to pull the tank in my 71 a couple years ago when doing cab mounts. The bottom and backside had lots of surface rust that couldn't be seen while tank was in place. I cleaned it, treated and painted it before it could become a more serious issue. So don't rule out the tank.

Also like Ray says, at the sending unit/fuel pickup there's an O-ring seal under the lock ring. Make sure everything's tight there. Check all the fuel line and rubber hose connections. Today's alcohol based gasoline is rough on rubber fuel hose, especially if it's original.