383 build for efficiency

It's not necessarily engine size that dictates fuel mileage. What I'm about to say is just generalized.

383 basically can get the similar mileage as anything other N/A is available for our A Bodies it's basically how much performance you willing to give up for mileage.

A 198 /6 is basically half the cid of a rebuild 383 so if you run a 383 at half the rpm at any given mph compared to the 198 /6 should get the similar mileage and performance for that matter.

Gearing and cam choices are gonna effect your mileage the most.

If your willing to go overdrive and custom roller cam with a wide lsa and short duration and maybe even high ratio rockers you could build best of both worlds with a good set of heads. Plus I'd stroke it out to 431 cid to be able to build a .9-1.1:1 hp per cid engine.

Just remember it's hard to recoup on mods for gas mileage sometimes.