Birge,
Where did you see the Felpro gasket listed for a 216/235/261?
There is no o-ring on my 1954 235s and 1960 261 distributors.
There are grooves in the shaft of the distributor, and I doubt if using an o-ring would cause any problems, but I think that an o-ring was not used on the distributor shaft.
Tonight, I'll check a Master Parts Book to see if an o-ring is listed (for a 216/235/261 engine).
Tim
Here is a listing of distributor parts Diagram of distributor parts
