Ron,
like you I have also thought outboard vents would be an improvement to cabin cooling. With 2511 laid up for extensive collision repairs I decided to pursue that modification.
in short, using the original HVAC blower/evaporator case it simply cannot be done. Due to the layout of the flappers and the defrost and floor outlets in the stock HVAC case there is no satisfactory location to tap into the dash vent system. One might be able to tap in to the floor vents or the defrost vents to provide air to outboard dashboard vents, but that sort of messes things up from a airflow distribution viewpoint.
what you see in the photos was only possible because I used an aftermarket HVAC system. The outlets on that unit needed extensive modification to provide dash, defrost and floor outlets compatible with a Pantera. I constructed a very large plenum that enclosed all three center dash outlets and the LH/RH ends of that plenum provided locations for ducting that could feed outboard vents.
I had to modify the glove box support structure to allow space for flex hose feeding the RH vent and had to relocate dash switches to provide space for the LH vent. There were also ...many... other obstacles that had to be overcome to bring this project to a satisfactory conclusion.
I am unaware of any other owner that has added outboard vents, and I now understand why this is so. It is an unbelievably time consuming and problem-solving project. Installing and then removing the dashboard over 100 times was just one aspect of the project.
unless you are willing to undertake a huge project my suggestion to you is to add the third vent, which I also added to 2511. I was able to add that vent (and the outboard vents) without reupholstering the dashboard with some very careful layout and execution.
I will be happy to share my third vent modification should you decide to go that direction.
BTW - I have yet to test the system for efficient airflow and may well have to design and install some diverter plates inside the large center plenum to effectively direct airflow to the outboard vents. Quite frankly, that should be one of the easier aspects of the project.
Larry