This rear end measures identical to the 64-66 mustangs. 67-73 are a little longer plate to plate. The spring perches are all the same. So, yes you can bolt that rear end under your '68. The spring perches are the same. You do get about 1 to 1.5 more inches of back spacing for your wheels because it is a little shorter plate to plate like the 64-66 mustangs. Now you get to change your MC, so you can run 4-wheel disc brakes. If the MC from the donor car is still available--HMMMM.
Ponyexpress thans for the info, I have been running the Versailles rearend since the late 80's in my 65. I seem to recall a rectangular gasket/seal that came off the original Versailles that I did not install. Not sure if it's needed, that's why I am looking for an exploded view of all the parts.