Ford wise, they where a predicted option for the 69' Cougar Eliminator--So far as I know, there were no production units, just prototypes. Pretty much a one off design.
There were aftermarket add-on's that attempted to do the same thing and were available before the Eliminator. They were called Hone-O-Drives and they were available to be mounted on the front of a 9", but most were added inline with the driveshafts. They really weren't strong enough for a hard running performance vehicle and most were installed in RV's.
Here is a Hone-O-Drive mounted to a 9":
Most people now look to Gear Vendors for similar tranny mounted gearboxes or just install a modern automatic/manual transmission that came with overdrive from the factory.
My little 65' Stang street car:
Dart 428W NA, 4 Spd, 4:33, on pump gas pushing 3550lbs.
email@example.com with a 1.466 60'