IMO something is off somewhere or the car is very, very heavy. The converter will help. I ran a 9 1/2" behind my Edelbrock (ported, mid 290's) flat hydraulic cammed 340, 4.30 gears. At 3240# it ran 7.24 at 94 mph with 1.54 60'.
I built a 408 using the heads off the 340 and had the converter configured for the combo. It was 10.8-1, 260/264 @ .050 flat solid, 4.10 gears. At 3260# it ran 6.57 at 104 with a best 1.38 60'. After about two years I killed the 9 1/2" converter and had an 8" built for the 408 and never got near the best 60'. Generally high 1.40's.
