You're right on with the larger 6.5" measurement. The Happ retainer goes on the outside of the top and bottom wood pieces and so the the marquee will rest on the bottom retainer piece approximately even with the bottom 3/4" wood piece (does that make sense?). The lip on the retainer does cover up some of the marquee artwork as you said, so you will lose a little bit of it behind the lip when mounted. I can measure that amount of lip space if you want when I get home. Personally, I would make it a tad under 6.5" (maybe 6.25") to make sure it's not too tall, yet still plenty high enough to be covered by the marquee retainers.