Most guys with stock heads shift 5800-6000, and advancing the timing would help a bit on the top end. Might kill some low end torque, so launch hard. I usually use the "highway feel" in 3rd (AOD) to see when it starts to feel like it's starving for air, and make ~250rpm lower a shift point. A chassis dynamometer would be best, but I'm a cheap bastage at this point.
Is everything else stock?