Tuning a DD 383 for optimum mileage / 1/4 mile performance

I have a 81 Club cab dually with a 400 big block and automatic transmission getting 10 miles to the gallon. I added a 6AL and picked up two miles to the gallon. It has a stock 76, 400 with headers and a 1406 carb. It eats malibu chettys for some reason from a standing start or on the top end.