I had a similar built 408 (was a Blueprint Engines build) in my 70 Duster with a 3000 stall converter & 3.55 gears. Pulled like nobody's business and at highway speed got around 13MPG. You should be fine with 3000-3500 converter with those gears, as it will cause the engine to flash to its powerband when you stomp it. I got the converter from PTC.