Since you know the cfm, you must have been here already,
http://www.wixfilters.com/lookup/FilterBySize.aspx , if not, it's a really good way to find a filter. You are right to be concerned about the cfm of the filter, it will work for a while, but will get dirty much faster due to the limited area of filter material. You need a much larger cfm ratting if you wish to use it for more then a few months at a time. I see those small 3" or 4" filters all the time at shows and just think to myself how much better the engines would run with a real life sized element! I have seen 454's with dual quad carbs and twin 4" filters, who in the right mind ever thought that was good idea, and on top of that, they look so out of place.
Look at the #42241, it's for a Mack truck, 275 cfm . You may have to modify the top, but it's a much better filter and not much bigger then a 4"x8".
