undy,
Ran into the same problem with the 521 in the Cobra and the 557 in the Zephyr wagon.
In the Cobra the solution was to increase the height of the scoop by about an inch. That got me enough airflow by allowing another inch of height for the filter. I can understand if you don't want to do that.
In the Zephyr the solution was a drop-base air filter. It allows plenty of flow without too much disturbance. Alas, it resulted in shrouding the float bowls of the 950 carb, creating a percolation problem particularly after the fully warmed up engine is shut off. Still working on that one - may have to let the fans run after the engine stops to solve it.
So your problem is a tough one - if you put enough boundary conditions on a problem you may find you've ruled out all the possible solutions...
One of the potential solutions I have not tried is the dual snorkel filter housing, like this:
Good luck and let us know if your solution works!
Tom