Mercon V is not necessarily synthetic. Most of them that I've seen are only synthetic blend, and I believe that even the Motorcraft stuff is synthetic blend. That being said, Mercon V IS the replacement for regular Mercon. Ford considers regular Mercon to be obsolete and says that Mercon V supersedes it. So any transmission that originally specified regular Mercon should be able to use Mercon V no problem.
And Dexron III is/was the exact same thing as regular Mercon, which is why you always see bottles of ATF that say "Dexron III/Mercon" or "Dex/Merc" for short. "Dexron" is a registered trademark of GM, and Mercon is a registered trademark of Ford. They literally had different names for the same fluid. So it stands to reason that any transmission that originally specified Dexron III could also use Mercon V.
Now, to further confuse matters, Dexron III is also an obsolete spec. GM replaced it with Dexron VI and says to use Dex VI in all transmissions that originally called for Dex III.
Even though the old Dex III was the same as regular Mercon, Dexron VI is NOT the same as Mercon V.
So, if you want to use a "modern"/latest spec fluid, should you use Mercon V or Dex VI?
Your call I suppose. Personally, I'd just use a generic "Dex/Merc" fluid and call it a day. There's plenty of selection, from a cheap conventional store brand, to a full-synthetic name brand. They'll all likely work just fine in your vehicles.