The valves and seats will be fine after sitting. I doubt that there is any problem there.
This likely is all just a bad carburetor. Get the engine running and hooked up on the 4150/4160. Then work on the new carb.
You do realize that you must set the fuel level on the new carb? The factory procedure puts the level too high and will flow out of the sight plugs as delivered new. That is better with gasolene not containing Ethanol. Here we generally have 10% Ethanol in the gas and it tends to expand the fuel level. At that factory set level, the carburetor will flood the engine.
You want the level just under the sight plug hole. I use an electric fuel pump to provide the pressure with the engine off/ Also you do not want more then 5 psi pressure.
The Pantera should also use an angled spacer under the carb. I think it is 12°? In US built cars the engines were installed in that era on an angle. The intake maniolds were angle cut under the carb to make it sit level.
The drive train in the Pantera unlike the US cars is installed level, so you need to compensate with the angled spacer under the carb to make the carb sit level. I mention that here because that does effect the fuel level adjustment in the fuel bowls.