Also as a rebuild, that means the clutch cover (pressure plate) was apart and then reassembled.
As with Chevys, these engines are now old enough to be third, forth and fifth generation aftermarket clutches. The retailers mix parts just like Chevy hot rodders do. I personally don't see that as a good thing.
If you are using a special unit like the Centerforce dual friction, the nature of the disc linings is going to permit them to slip until they are "fully broken in". Centerforce says that is a MINIMUM of 500 miles.
It is considered capable of holding 800hp. But that is fully broken in.
It's kind of like using full race brake pads on the street. Those things will feel like the brakes are hard as bricks and they are under water as far as braking ability until they are red hot. In other words they are slipping, just like the Centerforce lining is.
I can personally tell you that the Centerforce will slip more on the steel flywheel then the iron one. Also. If you installed a new aluminum flywheel, those things, depending on who's it was, probably need to be cut new, out of the box, to make them flat.
If it is some other kind of a "Spec III" puck design disc, then good luck on that one. I'm not even sure how you make that one work in a Pantera at all. Particularly considering the possibilities of mixing components here such as the disc with or without Marcel. The iron, steel or aluminum flywheel, and the lack of travel on the slave cylinder.
Best to stay with a special Pantera ONLY clutch assembly and leave the clucth "experts" go play with their Camaros, Mustangs or themselves.
As far as the throwout bearing being machined concave, that's the first I've heard of that one, but that will not make any significant differences. Don't even look there.
The 40 thousands minimum clearance is actually a little tight. 50 would be better. That usually would have shown up already in a transmission that is grinding between gears and is difficult to push the lever into gear (particularly second) without grinding.
40 may be all that you can get with the Pantera design. 50 really as I recall is unobtainium in a Pantera even with the so called "long travel" slave cylinder.
Remember also that the original Pantera clutch disc has no Marcel in it. It IS NOT the same as a CJ Mustang. But that has to do with giving the slave cylinder more travel.
My bet is that this is not a fully broken in clutch and that there isn't anything wrong with that at all.
Put at least 1,500 miles on it and see if it gets better.