First off, you need to verify that the pressure cap is sealing. 1) pressure test the cap itself 2) verify that it is the correct cap for your pressure tank 3) pressure test the system
This is a great opportunity to get a Stant pressure test kit. It's a little over $100 but is really essential to a car like a Pantera.
IT IS common for issues with a mismatched pressure cap and the pressure cap. The tank itself is a European design on the neck and does not use a US style cap.
Also, when you use the pressure kit, you will HEAR the air pockets being pumped out of the system. Sounds of gurgling are sure signs of air pockets.
Without pressurizing the system manually, you virtually need to stand the car on it's nose to get the air out.
The head gaskets are marked "front" on them. Since they are both the same, that means one needs to be turned upside down with the marking towards the block.
Yes you can pull the heads with the engine in the car but to work on the engine you will need to remove the decklid and climb into the engine compartment.
If you use head studs on the engine, that's another issue all together and likely not be able to get the heads off with them in place.
You do not want head studs on an Pantera engine.