I myself was able to get 19/20 MPG's out of a 360 in a '79 Cordoba.
I only did bolt on stuff. An "H" piped 2-1/4 exhaust off the exhaust manifolds, 625 Carter on top of a OE intake, a K&N filter in the factory air cleaner, a vacuum advance distributor from MoPar and a MSD 6A box.
Gears were 2.76 and tires were 235/60/15's on all 4 corners.
Tune it well and cruise it easy. Enjoy!
