Not a Mustang or Super Stocker, but here's something I made for one of my personal hotrods. It's a 3pc setup where the bell, block plate, and midplate are all separate pieces. This gives me the additional option of removing the complete engine/flywheel/clutch as an assembly without having to disturb the trans/driveshaft/shifter. Having the support ears "up" rather than "out" keeps them completely away from the headers. In this pic you can also see three added bolts that serve to keep the bell attached to the midplate when the bellhousing bolts are removed...
The car does have a crossmember under the rear of the trans, but it serves more as lower support and fore/aft travel limiter, it does not bolt to the trans itself.
Here's a pic of the block plate installed on the engine, I made the center hole bigger to aid clutch cooling...
Here's a pic from the inside showing one of the aluminum brackets I made to spread the loads across a wider area of the car's thin sheetmetal firewall...
Like I said this is not a Mustang, but it is similar in that it is a unibody car. This car has had the midplate attached to the firewall for 12 years now, no problems.
Here's a link to some info about the rest of the car, there are some pictures on there that document my process of fabricating this bellhousing from scratch...
http://grannys.tripod.com/20102.html