It's exactly what Chuck said Dusterboy. You have to determine whether you've got a forged crank or cast crank before you go any further and this can only be done by removing the pan. I'll attach some pictures below that make it easy to tell and you can also run the casting number off the crank http://www.moparts.com/Tech/Archive/bb/22.html This also a picture that shows the different vibration dampeners. http://www.moparts.com/Tech/Archive/motor/8.html
and casting numbers for cranks