Streetable 12 second 20+ mpg slant six?

I'm not trying to argue either. But Everyone seems to think small engines is the only way to get mileage but it take a certain amount of fuel to get your car moving (weight) no matter the engine size. 20 mpg in town is a hard number to get even a lot of modern cars (similar weight) with all the tricks. I hated the lack of power my /6 had off the lights in town. If the OP wants to go the/6 route it's up to him just trying to say in the end he may only gain a couple mpg over a 360 but at a huge lost of performance and 273 would probably be the same mpg but have a decent bump in power.