V6 Fi Vs 318

I have a 74 duster 225 slant six. I put on headers and a 4brl intake and 390 cfm carb. I can only achieve 17 mpg at best driving the car 55 on a flat road. Would I be better off, money wise and mileage
wise, trading up to a 318 or turning my slant six into fuel injection? Thanks!!!Tom