i just re pounded mine too to the other day. its no fun with the headers installed. i used a 12 inch or so metal shaft with nuts on both sides. all i had was a roofing hammer, or a 10 lb little sledge hammer. baby sledge did the trick. but killed my arm.

i saw a few vids of guys with headers and in reverse you can hear the o2 hitting. mine made the same sounds till i re pounded it better.