To convert Newtons to Nm from a Dyno reading, you need to know the roller radius, which is 108.5mm on the dyno mentioned.
Just muliply 6000 times .1085, and this will give you your peak Nm reading.
Your car had a maximum Nm reading of 651Nm in this case.
Newtons, or Motive Force which is the correct name, is the force at the tyre diameter pushing the car forward.