Internal or External Balance?

Some Mopar engine balance is way off from the factory. Does the motor have the stock pistons? Some aftermarket pistons are lighter than stock so it can upset the factory balance. Did you purchase the flywheel new or used? '73 340 were external balance but they had a different balance so possibly your flywheel is a cast 340 flywheel? what diameter is the three holes on the back? better yet what is the casting number on the flywheel? this would tell you for sure.

Chuck