Streetable 12 second 20+ mpg slant six?

Lots of /6 turbo info out there, you can meet your goals easily. Basically what you're gonna need is:

Rebuilt short block
Valve job and scrub the ports out of slag
Possibly bigger valves
RV cam
A heavy wall log manifold would be best
Match a turbo to your cid and rpm
Carb works well or you can go FI for more reliable power
Gearing in the 3:23 range