Yes the spacers can be used to a degree to tune the manifold.
The biggest reason to use the Blue Thunder manifold on the Pantera is that it is very close in height to the stock iron manifold.
That means that the entire assembly of manifold, carb and air cleaner will fit under the rear engine cover that the Pantera has.
A 2" high spacer would push them well past the top of that cover.
If one decided on that route than there are a bunch of much better manifolds to pick than the Blue Thunder.
The car does definitely look cleaner, i.e., better from the rear with the original cover over the engine.
Of course I'm partial to a gaggle of Weber carbs hanging out the back.
Some would akin this to a poor woman that has to bear the burden of a DD top with her cups constantly running over and find that very tacky?