What do you have for jets and what is the finish line RPM? The peak tourque looks to be at 5500 so I would want a 9" converter at 300 over that, and where was the timing on the dyno? Is the converter able to be be efficient at the finish line? Did you move it around? I too think a 456 or more gear would help, but maybe a 10th, and it should pick up more with the tune up. I don't think 870HP is anywhere near an 8.50 in a 3000# car. 8.80-8.90@ around 148-150 would be my guess. JMO these are the questions you need to answer.