I have recently completed a webpage which includes many early records for both of these lines and establishes that there was definitely some connection between them. My research is based solidly on records with scans of quite a few of the original records. You can find it at http://freepages.genealogy.rootsweb.com/~fcharper/mckupdate.... If you can connect your McKeithan line to either of these earliest McKeithans, I think you'll find the records here invaluable.

I'd very much appreciate feedback on this work.