What about timing, total advance, advance curve and vacuum advance, play a huge roll in fuel efficiency. Is your total timing 40 degrees+ when vacuum is in?
What is your cruise RPM?

My duster with a 422ci small block 750BG double pumper,3.23s gears,.82OD trans and 28inch BFG drag radials gets 19 to 21mpg @ 70mph. Around town not so much but not bad enough that I check town driving mpg. Timing Vacuum advance in is 48 total.