You've got 54" to work with, and 3" is not very much for molding at all. Do 30" with 18" on top, with 6" of stacked molding if you have a 9' ceiling. That's the standard height for transom wall cabinets which will be cheaper than installing stacked cabinets, both from a materials and labor standpoint. That's the proportion here in my Kemper display. The cove molding needed a bit of a riser to make the 9' ceiling, but it was close enough for a display vignette.
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