funny.. roughly my dimensions. I would think a 54 or 56 would fit, which likely makes that a 58 or 60?

Should be able to look at Vintage Cannondale - Information and Catalogs for Cannondales and find rough geometry chart to confirm.

Keep the 7800 dura ace, find a used CAAD frame on sale, have a shop swap parts over (or if you're mechanically inclined, do it yourself).