14.5 MPG 350/700r/3.73's at 70 to 75 MPH - Should I be happy with that?

[stomper]

Did a 160 mi trip this weekend over a mountain pass and into Eastern Washington. AC on a little while. Do I have room for improvement or should I be happy with 14.5 MPG.

Details

'71 short fleet, lowered 4/7.5. Only payload was; me, the dog, a cooler, and emergency tools.

Stockish unknown miles '69 350 (.030 over) 140-150 psi compression.
Rebuilt Q-jet, Edelbrock SP2P intake manifold, tall open element air cleaner.
2" Ramhorns and 2-1/2" exhaust.
15 in vacuum on stock gauge at smooth idle.
Running 2200 - 2500 rpm at 75.
All new ignition. Summit HEI with one lighter advance spring - 14 degrees base timing. Vacuum advance can left as delivered from Summit.
Tune seems good (snappy and doesn't ping) but I feel the grunt falls off above 60-65 MPH.

Swap meet 700r. Lock-up doesn't want to stay locked-up after a few miles on the highway and rarely above 60 mph. I have a new solenoid to go in but that's another story.

Craigslist 3.73 posi - counted the teeth to confirm ratio.

40 series 20" tires - 29" tall. New and aligned. Didn't check air pressure.

Speed GPS confirmed.

87 octane 7-11 gas.

After reading some posts I'm thinking a couple more MPG could be expected. What do you think?