The stock air filter setup is not that restrictive. The "mushroom" design with domed lid actually enhances air flow. If anything is a restriction, it would be the rear opening of the shaker. My WFO unit for '73-'76 shakers uses a flat panel filter which is necessary for the low-angle scoops The filter measures 6"x9" and adequately supports a 500hp engine. I have several customers who run roller cams and edelbrock heads and have no complaints of air flow restriction or lack of performance.
Click on the link and browse the photos for the '73-'76 shakers.
http://community.webshots.com/album/129263156jDeXOf