There is a "core" existing that could be used for a rear rotor.
If you look at a GM '98 Blazer rear rotor, that would be the core.
Now I call it a core simply because it isn't a simple bolt on. It is the thicker rotor, i.e., .970" and it is 12.3"od. It uses a GM bolt pattern, not the Ford 4-1/2" center. It is vented. The center hole will clear the Pantera's center hub.
What is significant though, is that it has the necessary interior clearance to clear the o.d. of the Pantera rear hub, and the necessary set back to mount externally.
The bolt pattern can be redrilled to the Pantera pattern and the od can be turned down to what you need but the rotor thickness is probably going to be the main issue to deal with if you want to keep your calipers?
So any machining necessary would be done on the rotor and not on the Pantera.
Because it has the necessary depth to sit over the Pantera's flange it has the ability to be shimmed to center it in combination of shimming the location of the calipers.
I am still looking for that rotor in the thinner .847" thickness but it does not appear that there is one that exists but it is a start and for those of you that are sourcing hunters, it could be the beginning of a discussion with a brake rotor company.
I just haven't found a cross reference of rotor sizes yet that I can search an application for. Most cross references are for "original applications" and don't list the critical dimensions of the parts. That's the cross references that I need to find the thing, if it already exists, which is probably a 50/50 thing?
Being a common GM application helps with availability and pricing.
What I did on my brakes was already posted so I won't bore you all with repeating that but I would like to find a rotor that installs over the Pantera's rear flange, so that's why I keep looking. "You never know" and you won't find it if you stop looking? There is no doubt that something will need to be done with the rear studs but that is likely a minor change.