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I am a little late to the party.  But, our Mach 1 had its original 2.79:1 ratio rear axle gears replaced by the prior owner with a 9" housing and 3.5:1 TractionLok rear axle gear set.  I have come to feel that the 3.5:1 ratio is about the lowest ratio I would want to go with a street driven vehicle, and at that the engine RPM as just under 2,900 at 60 MPH.  It was tolerable, but still far higher than I am used to with our other First Generation pony cars.  So, in a bid to reduce the engine RPM when cruising, while retaining the  lower end performance of the 3.5:1 gears, we opted to replace the original c4 with an AOD.  I have a suite of YouTube videos, to include videos of the actual replacement itself, under my YouTube channel (Gilbert Hale).  In the description of some of those videos I have links to various vendoeds and the parts list used.  I am now spinning the engine at around 1,900 rPM at 60 MPH, and have recaptured the upper end performance as the street/strip built engine has plenty of torque to push through the air resistance at triple digit speeds.  The OverDrive has provided us with the best of both worlds.  I am veyr glad I did it.  For that matter, I am betting the AOD would have also nicely tamed even a 4.11:1 ratio gear set, if I had them.  I am a happy camper.