There very well could be some improvement in gas mileage in a smog dog low compression engine using a turbo in a very low boost setup with 6-7 lbs.
Just my two cents but similar efficiency gains could also be had with a set of pistons to bump compression and set up for quench.
Second option is by far the simpler way to go about it imo.
If mileage is the primary concern I would start with a 318 v/s the 360.
The engine will not be under any boost the majority of the time, it will still be under vacuum under most driving conditions except now it will be working harder to pump air and exhaust through a pretty big restriction.
