the 318 will definitely get better fuel economy over the 360 in different applications. your problems will be, the weight of that RC,the gears,no overdrive,more bulk of the 727 internals,less cubes(with the 318),all will combine to get less than stellar results in fuel economy. you will likely do all that work,and gain 1 mpg, or none.
i mean face it,your not driving a 4-cyl dakota. your driving a big heavy pig,with not-so-highway friendly gears,and a V-8. great mpg's are not looking good........