Slant Six mpg's?

i have been getting about 18.6, mostly around town. i have a super six with headers, auto trans, 2,76 rear and 195 70 14 tires, in a 4dr valiant.
i am also wondering what others are getting for mileage, i keep thiinking there is a few more mpg's in there. i have tried several things to improve mileage, the super six, cold ram air, HEI, NGK plugs, bigger exhaust. only the plugs showed a noticable improvement (1 mpg).
for mileage in your b body, you might be better off with the 360. i read somewhere that the 360 has the most fuel efficient combustion chambers of all the small blocks. probaly why it lasted longer than the rest