I would like to find a good motor mount for my 67 skylark with a 455.
Poston sells solid mounts, are they the best preformance option?

I would not go with solid mounts unless you have a race car with a cage and midplate. The solid mounts will allow the frame to apply force to the block. These forces can pull the block apart at the mount location unless you have reinforced the frame and chassis appropriately. In other words, the solid mounts allow the engine to become part of the chassis, and it will be subject to the forces acting on the chassis.

You may want to consider using a tether in conjunction with rubber mounts if you want to prevent the motor from rotating in place. This will save the mounts and provide a more consistent launch at the track. The tether can be a length of chain or cable attached from the drives side head to the frame. This prevents the motor from rotating toward the passenger side while not affixing the motor rigidly to the chassis.