You are getting castor mixed up with camber. There is no castor effect on the rear.
On the rear there is adjustment for camber but unfortunately it's only adjustable in the wrong direction. The top arm is fixed and the bottom arm is adjustable with shims.
A typical Pantera with all it's lower arm adjustment used up (all possible shims removed while still maintaining the correct toe in) will have approx 1.0 -1.5 degrees negative camber.
Negative camber means the wheel is leaning in at the top. My car (with non adjustable arms) has 1.5 degrees. This is not excessive but it does mean the inside of my rear tyre wears down a bit quicker than the outside.
The wider the rear tyre the more critical camber becomes. I think the De Tomaso factory specs for a GT5 where 0 degrees on the back. This is only possible with an adjustable or extended top arm. I have installed some very nice adjustable top arms on my friends GT5, we bought them from Roland Jaeckel in Germany email@example.com This gives us all the adjustment we could ever need. For example if we wanted to do a track day with the car we could dial in some more negative camber for aggressive driving and then wind it back for driving on the road. I personally like to have 1 degree negative camber on the rear for a road car.