318,340 or 360?

hey im trying to come up with a plan for a duster as im getting some cash layin around soon not much but enough to get started and im tryin to inspire my dad to help out but i need more info for the engine and trans combination he wants it to stay in the mild performance range and get good milage but he doesnt want a /6 i want good performance(300hp-400hp) at least 13mpg as i will be driving it to school every day ay advice on builds would be awesome i am open to anything id like to keep costs down but will consider more expesive builds for the future thanks