Hummm....give me a full 8.25" to work with width wise and things get very interesting!
Assuming that width, I can do a WWMTMWW for the center and go with quad Dayton RS 7's for the woofers and still use the RS 6's for the mids. This would be a step up and gives us similar cabinet depth while increasing height to around 48". I really like this idea as the 7's are a step up from the 6's and I think would serve you even better.