273 spark plug mishap

There are no good reasons to use the wrong spark plug. Could have bought the incorrect plugs or because of oil or fuel fouling. That is an excellent question to ask the previous owner - please let us know the answer.

There are many factors that go into the dimensions and specs of the plugs. I can't say anything specific about that particular plug but we can assume it was for a much different application.

How did the car run? Apparently there was enough spark to ignite the fuel air mixture but that doesn't mean it ran as good as possible. An improvement in fuel economy, performance, idle and starting might be seen with the proper plugs and tune-up.