360 driver build
OK, at least a 9.0-1 comp ratio
standard 1.88 360 heads
600 cfm 4bbl
headers with a good exhaust system
Comp Cams XE262H
The issue with this is the cam will need (According to Comp) at least a 3.23 gear. Not exactly a 20 mpg freindly gear. A lower duration cam will help mileage but hurt HP. I've seen guys get a cam like this and larger and add Rhoads lifters to try and lower the operating RPM range of the cam/engine with there Hwy. gears.
Add a MSD type like system for a good mileage return over the stock system.