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.