It likely fits down into the gas tank through the filler bung but it would have to fit tanks that had easy access from the top like the Model T, A, or B. That would be the only way it could work as a dip stick. It also would likely slide up out of the tank bung a ways to act as a funnel to a certain degree although if it remains in the tank it would act as a baffle for filling but that's only if fuel can pass straight through. Whether it would work well depends on how well it fits. Not all Ford fuel tanks have a straight entry way in. The later cars had filler tubes interfaced with the fenders and some are curved.
Since the photo of the thing doesn't show but a side view, we can't tell if it has a bottom in it or if has anything else to it that can't be seen. If it is just a can with a bottom to it the it would have to come out to fill the tank. It would be the only way it could hold reserve fuel but it wouldn't hold much. Some contraptions like that are more hassle than they are worth and you would end up smelling like gas at every fuel stop whether you wanted to or not.