You might need more converter, you put bigger heads on that raised the torque curve most likely. Then you put bigger headers on doing the same thing. It might need to leave at more RPM.

I changed converters and rear gear at the same time, from 4500 to 4800 and from 4.56 to 4.10. The flash RPM went up almost 800rpm the 60ft dropped .08 to a best of 1.41. 416" W2 motor in a 3550lb A body. That 60ft was on a 6.63pass@101.3