318 automatic, 3 speed, slant six, your perfect choice for a daily driver

my 74 duster slant six is getting 18 mpg mixed driving, that is city highway and putting it to the floor at least 2x per day. i put the super six and a 2.5" exhaust on it. the exhaust smells rich i think it will get 20 when i work the bugs out. it really isnt that bad for power between the ported exhaust manifold with opened up outlet, the 78 2 brl carb and exhaust. it made quite the difference in power. its an auto, i bet a manual od trans would help.