The plenum shown is a viable solution for a "street driven car", no question, BUT anytime you go to a IR induction, you change the "sensitivity" of the signal to each intake.
When you enclose them, you have altered that sensitivity again.
The stacks just do not like any kind of a flow restriction.
The plenum that you are creating there, over and around the stacks, needs to be "tuned".
It is possible that this one pictured is right on the money but the answer to that is really that it will vary according to the rpm target of the engine.
In other words, the size of that plenum may be perfect for an engine operating up to 5,000-5,500 rpm and then it gets restrictive and operates as an rpm limiter?
We clearly can see that on the Ford COBRA oval air cleaners on the carburetored induction systems where beginning at about 5,000 rpm, those air cleaners start the restriction and virtually can't flow enough to go much past 5,500rpm.
That is easily shown by opening the air cleaner and just leaving the base plate installed. Then you can rev to the rpm limit of the engine.
A simple way to detect a change would be an improvement in the 1/4 mile time of the same car. That resulting air cleaner change would show a horsepower gain in the 80 to 100 hp vicinity, depending on track conditions such as traction but make no mistake, that really is a large improvement just by unrestricting the engine induction with no other changes.
As I recall, just taking the air cleaner off of a pretty much stock 428cj Mustang results in an improvement in 1/4 mile times of about .3 seconds faster time.
That is exactly what has been done here. The air box has created a restriction.
To run something like "the Silver State" event, you will need to take the top off of that "air cleaner assembly".
In the case of putting air cleaners on the Weber 48ida system on a V-8 engine cam'd with anything more then about 28° of "overlap", you WILL get fuel reversion that WILL soak the air filter element and IT WILL catch fire on HOT ENGINE restart.
With EFI you can put air filters on the stacks, because with an IR EFI there is no atomized fuel retained in suspension in the "venturi" to be pushed out in the instant reversed flow, since the fuel is "squirted" directly into the intake ports.
So the simplest thing to do in a Pantera is to use tall stacks that come within the minimum clearance over the top of the stacks (which is the outside diameter of the stack) and the roof of the engine compartment and screen the stack using the largest surface area of screen over the stack that you can fit in.
That is going to be the best combination of compromise you can get.
Additionally, the "bell" on the velocity stacks isn't there for decoration. It is there to work like an "Event Horizon" on a "black hole", accelerating the incoming air for more ram effect.
If you block that, you negate that effect.
The GT40's in the day running "Webers" just put a "cookie tray" over the carb stacks for two reasons. First, It prevented anything large from falling into the stacks and secondly, it provided a way to control the reversion fuel droplets from accumulating uncontrollably all over the engine compartment including the rear window and the bundle of snakes headers.
On the GT40, the entire rear engine compartment is the "air cleaner"
If you are building an air box of any kind over the stacks, you are just building an induction system for show, and not go. That is the reality.
It is going to have induction restriction. The only question is where in the rpm range it begins to appear and how much it restricts the engines top rpm capability?