Streetable 12 second 20+ mpg slant six?

Not without turbo or some kind of power adder.

Slant 6 head is the weakest part, untouched your lucky to make anyway near 200 hp ported could make up to 300 hp but to get mileage you want to be in the 0.8:1-1:1 hp:cid which is 200-225 hp over 250 hp you'd start to have more of a street strip engine.