[Jack21]

You don't need a 9" diff for a stroker 351W. An 8.8 will do just fine. Use 31 spline axles, and a decent aftermarket Posi. Put the money into the trans, rather than the diff.

However, If you feel you do need a 9", any of the aftermarket suppliers can set it up as a '79 - up Mustang replacement without the horizontal shock brackets.