noob needs help with mileage - only 10 MPG!

dropped down to 8-10 when I swapped in an 8.8 with 3.55, pulled it shortly afterward. It was a 77' super six. It had flow issues with the head and never ran properly.
must have had some kind of issues- I had better mileage...ok, about the same mileage...with my 340 and 4.10s.