[maxum_101]

my brothers car, with a 460 also has a clunk noise to it when hitting the gas, mind you.. its not a hard launch, just your standard take off from a stop.
upon closer inspection, we noticed that one of the rear end mounts had broke thru the mounting bracket. it has a T bird rear in it. its the mount that is on both sides of the rear end, passenger side.(looks like a small hockey puck wth a bolt thru it. its broken where the "hockey pucks" bolts to the frame. maybe check there to see it you have any cracks or it might just be broken thru like my brothers.
good way to check, car off.. put it in gear look under car toward rear end area, (bottom of diff and axels) and have someone rock the car front to back real good and see if anything is jumping around, like the rear diff or axel where they connect to the diff.