If your Pantera does not spring to life immediately when you switch the ignition to start, you might want to check out the ignition system wiring.
Early Panteras used a ballast resister between the ignition switch power and the ignition coil. Later cars used a resistive wire rather than a ballast resister. For some unknown reason on the early cars de Tomaso did not use the wire that normally connects from the plus side of the coil to the start output terminal of the starter solenoid (this terminal is the small terminal next to large terminal for the starter cable). When the ignition key is turned to the on position the switch applies voltage to the coil through the resistive wire or ballast. When the key is turned to start position and the starter solenoid energizes and the start output terminal applies full battery voltage to the coil though the wire. This direct voltage to the coil provides extra spark plug voltage for startup and compensates for the voltage drop in the power system while the starter motor is engaged.
If the starter solenoid does not apply battery voltage to the start output terminal, or if the wire is missing from the start output terminal to the coil the engine will exhibit difficult starting, e.g. the engine turns over many times before starting, starts when you let go of the key . Often the resistive wire on the newer cars has been replaced with a non-resistive wire and a ballast resister added near the coil. Make sure the ballast resister is wired correctly and the wire from the starter solenoid is attached on the coil side of the resister.
