A very close friend of mine just restored a 69 Z and bluprinted the motor. He saw a peak horse of 414 horse at 7250.
But it didn't start to climb untill about 4500. I have had two Z cars, and the 1st was a 50k original mile machine, and it hauled, from about 3500 on up to about 7500.
The problem with 1/4 times, is that you almost have to get out and push, to get the car rolling from a dead stop, regardless of the gear, and coming out of the hole at 6500 rpm isn't going to do much for traction.
My car is a factory Z with some minor head work, factory cam, and manifolds. IT has a 750 Barry Grant on it , and it tears like crazy. 400 is obviously not out of the question.