fwiw - I get 14.5mpg on the highway, corrected for tires/gearing, with a 351M, Edelbrock Performer, Holley 600, Dynomax Headers and 2-1/2" duals thru glasspacks, no cats and an open air cleaner. Pretty standard setup, with no vacuum advance and timing at 10 deg BTDC, and 93 octane gas. truck is lifted 2-1/2" with 35's and stock gears. It's easy to get road numbers below 10mpg with a heavy foot, and it gets 5-6mpg on the beach.

Dave
'79 Bronco
'89 Trooper