If you can find a 6.125 inch damper at a wrecking yard with a flat front you should be okay. A lot of 305's used the small one and they should be very readily available for about 10 bucks at a wrecking yard. Take your old one and if you can match size (diameter) you should be okay. The key is to have a flat face.

As to the spacer, you could try it but balance would be real important. If it's not perfect, the vibration could be significant.