I had a 70s 360 in my 67 Barracuda FB with an 833 o/d trans and 3.23s. I was getting 19 mpg mixed driving back and forth to work. I never did take it anywhere very far when I had the o/d, so no real highway numbers. I bet it would have done low 20s pretty easy.
Travis..

70 GTX project, orig 440-4, 4 spd, track pack, FC7, stripper/street racer special.