I had a similar situation when I purchased an underdrive crank pulley for my '66 383. Some dampers had a bolt pattern where all six holes were evenly separated "symetric" and others were "asymetric". My crank dampner was asymetric and my new pulley was symetric.

What I did was overlay the new pulley over the original pulley and rotated them until all holes lined up except for one. At that spot I elongated the hole (in the new pulley)
so that the last bolt will go through.