MPG Increace

If that is your set up in your sig, you are screwed for mileage.

Going from rear to front,

2.76 gears
Tall tires
2-1/2 exhaust max (IMO)
Get a stock 360 cam
600 CFM carb or a TQ w/small primaries
MSD or equal ignition

I was able to get 20 MPG's (at interstate road) with my '79 - 360 in a Dodge Magnum. It was the OE engine with these parts;

Trap door air cleaner w/a K&N filter
OE intake w/a 625 Carter
MSD and a cavuum advance distributor, plugs gapped @ ,055
Dual exhaust off of the exhaust manifolds w- a "H" pipe @ 2-1/4 inch pipe size and Thrish turbo mufflers. The law required cats. I used two before the "H" pipe.
2.76 gears and 235/60/15's.

Tune this well and lean on the cruise. You should at least get 18-19 on the Hwy. use of a taker tire than what I used (26 inches) should help a little bit.
When your meage seeking, every little bit helps the big picture. When you start to max out, the gains are hard to find and the changes found when dividing miles per tank into gallons used can become a fickle math problem.

Good luck!