The K&N filter people say their filter material is designed to pass 
air at the rate of 6cf/min so it's relatively easy to calculate the 
filter area and end up with a cfm rating for the filter. Now, the 
question is... how does one equate lbs/hr to cfm in order to come up 
with the right size filter?

Try this,

 

Your engine is an air pump.  For a 4 cylinder aircraft engine each revolution has 1 cylinder drawing in air, 1 compressing air, 1 firing and 1 exhausting combustion products.  Thus, 1 revolution consumes 1/4 x 360 cubic inches or 90 ci or .625 cubic feet. At 2700 rpm and WOT, that's 2700 x .675 cf or 1688 cfm.  Lbs/Hr depends on air density.

 

K&N has a formula for size:

 

filter area = (CI x RPM)/20839

 

filter area = (360 x 2700)/20839 = 47 square inches of effective filter area, add .75" to each dimension for actual since air doesn't flow well at the edge of the filter.