Large O ring on dizzy? Rubber or other

The large O-ring doesn't hold the hold-down plate; that plate is held to the distributor with a bolt and to the block with another bolt. The large and small rubber (yes, they're rubber) O-rings are oil seals. The small O-ring being narrower than the distributor shank groove is normal.

Sounds like you're due for new O-rings. Use good quality, oilproof ones. If you can find an auto parts store with a good selection of air conditioning O-rings (often green or blue) you can usually find the sizes you need.