You didn't say what gear you're running, but the 60' times are way off, don't know if you're spinning or the converter is bad, but MPH doesn't look bad for the 6.60 et.
I can only go as an example. We have a 3700lb Biscyne, with M/T street slicks, and a pump gas 468" motor. The last time out in street trim, minus mufflers, it went 6.87 @ 99mph and 10.88 @ 120mph. Best 60' was 1.447 worse was 1.513. Thats with 4.56 gears and turbo 400 leaving at 4200 on the trans brake with a 5500 converter. I don't believe the motor has much more than 600HP, the cam is only .668/.674 lift and the comp. is 9.7.
I'm thinking also that you have converter problem unless it's struggling off the line.
JMO
Zip.