My general guide is that its often preferred to add to an older thread if the question asks to confirm if the information provided years before is still believed to be correct and applies to the same focus as the original. Believe this is better over time to group the discussions together rather than have links in new subjects to multiple older threads and reduces the number of hits one would get with the search feature
If the question is about the same feature, part of process but a different year or plant and you can't find another thread with that focus start a new one, normally in the Mustangs by year area.
Areas like Restoring - General discussions are for discussions that span across many different groups of years and models. Example would be a discussion of exterior door handles that were used on multiple years and plants
There are those times were the discussion starts out as a focus on (for examples Nov 65 NJ) that expand outside of that month but we deal with those as logically as we can sometimes spreading out the focus or splitting the topic depending on how the discussion goes.
No prefect set of guidelines so we adjust and do the best we can trying to keep things organized given how detailed and specific the subject matter here is compared to other forums, in an effort to provide the best and most focused information we can
Hope this helps explain at least a little bit
