To gain some confidence.. I'd be inclined to put a vacuum gauge on the fuel pump and check to see how much vacuum the pump can pull. Then build the overflow tank, fill it, and use a hand held vacuum pump to see how much vacuum is needed to evacuate the overflow tank. You'd want a length of rubber hose about the same as you'll be using on your truck... and the hose should be held up above the expansion tank, at about the same height as what it would be in the truck. As long as the vacuum to empty the overflow tank is less than the vacuum the fuel pump can generate.. your good to go.