Are you swapping big inch to little inch?

Aunt had a Chevy Kingswood Estate Wagon with the 400 SB 4bbl and 400 trans. It got 14 MPG on a long road trip.
Her new husband was a real Fart Smeller and decided to put his 283 2bbl and Power Glide in it for a road trip from Seattle to Saint Louis. It got the best mileage when they went thru Nebraska, 9 MPG but averaged a wopping 7 MPG for the trip. They could not even do the "Speed limit" back then.

If I had a 1970-75 A body and wanted to do good gas mileage but still have a lot of fun, a 4g63 2.0L with a BIG hair drier mated to a Baby 904 from a 1978-83 Sapparo or Challenger.