Papadisco,
Number of teeth on the starter is the same on a 6 volt starter as it is for a 12 volt starter, but they are different and cause grinding when mismatched. You may have the wrong starter.
How do you tell? I do have a 12V upgraded system, but the engine is also a mid 50's 261, so it might have been 6 or 12V originally (and the flywheel that went with it).
