Rear gear recommendation for highway.....

I am rocking a 318 with a pretty big cam in my 73 Dart Sport, been driving it daily this summer. 904 w/a 3500 tci convertor, 3.23's and 27" tires out back. 3k rpm at 65, it works great for the straight open back roads around here. Best I have gotten is 15.9mpg, but I usually get 13-14 with my heavy foot, gas is cheap right now. :D