The problem with dropping the base is, a big drop in cfm.
On the chassis dyno, we tested the Stelling/Helling cleaners, my oval cobra cleaner, typical 14" round and a drop base 14" assembly. Both the Stelling and the drop base choked way too much air. The dropped was the worst, because half of the filter was not above the carb seat, so it was worthless. I'll see if I still have the numbers of the test.
I too would like to find a answer for a drop base as I also have a clearance problem using the Victor single plane manifold, so I to milled down the carb surface 1/2", but I'm not going to harm the motors performance by loosing cfm. Speaking with a few engine builders, the 1/2" will not have much affect, if any, on performance.
Just for knowledge, with the K & N style oval air filter, the worst thing we did was over
oil the the filter, thus loosing cfm, a very light coverage was fine. To our surprise, a regular paper filter supplied a little more cfm than the K & N.
When oiled properly, they will trap more debris than the paper, just don't over
oil them.