Did you order the Male (HEI) or Female cap? The Male cap does not fit under the screen. I am running the Male cap on my 74 Euro Pantera and it does not have a screen but it clears the firewall and does not misfire. With a Female cap, we put it into a customer car and it fit under the screen. The cap is a standard GM/Chevy cap and it is readily available in the US at Autozone in either form for not much. It might be harder to get in France.
Here is a photo that I just snapped of mine with the Male HEI cap.