If I were building a 318 for mileage it would have to have a zero deck piston for good quench (like the KB), the late model #302 casting heads (cheap to find) with a little bowl work and a slightly ported SP2P and a 600 holley vacuum with the old redline/weber 3 circuit metering block and a lock-up 998 trans or a manual od 833 with a final drive ratio of around 2.94. The cam would be the old Cam Dynamics 252 which was (IIRC) around 206 @.050, .440 lift single pattern on 112 centers or even the factory 360 2 bbl cam (.410 lift.)

The sp2p was really a great concept, it's just that the port cross section was made about 10% too small...which made it detonation prone at high load and part throttle with 70's style emissions, it'd be perfect on a 273 built the same way.....I'm thinking I could build a 273 that would pull 65mph at less than 2000rpm and get at least high 20's mpg in an A body....or about the same as a 90's era Maxima v6. Keeping it very cool (like under 175 degrees) will go a long way to making the combo work on pump gas too.

I've been looking for an sp2p to play with for such a project.

the LA motor is one of the most fuel efficient v8s made for its time