Strange Transmission Problem

Check the linkage adjustment

I've already adjusted the linkage properly. Also I don't see how a poorly adjusted linkage could be causing a reverse in the order of gears.

Will you verify you are talking about a classic Mustang! I have never heard of a column shift Mustang.

Generally if there are shifting problems the first things to check are: 1) fluid level, 2) modulator and vacuum lines, 3) linkage. From there it gets into internal stuff.

I haven't seen a column shift Mustang either. As for the C6, what you are describing for symptoms, is bizarre, as the flow paths for the clutch packs is directed through the selector valve through various other valves and eventually to the appropriate clutch pack. So for it to come up with a new path, would mean something has happened to the valve body. And the way I have always adjusted transmission selector is to disconnect it from the trans at some point and in neutral, have the transmission selector in neutral and see if they line back up and slip together, if they do, then connect them up and check that operation is okay in all the gears and park. Then the neutral safety switch may have to be adjusted also. Now keep in mind, I am not a transmission specialist, just an old mechanic. My 2 cts. Good Luck.