I'm not positive but I think you are mistaken.
The units are both single Din but with larger fascias. If anything I'd have said the CMD6 is the bigger unit. Either way. the CMD6 should cover the hole and allow you to bolt it in so shouldn't be an issue.
If it were me I'd look towards the Fusion MS-IP600. I've been working with that stereo and its predessor for a few years now and do not believe there is a better unit on the market. it has independend zone control (so you can contorl sub, tower and in boat separately.. all with individual setup controls) and lets you slide an ipod or iPhone right inside. If its an iphone it'll ring through the speakers if someone calls too which is always useful on a noisy boat.
In over 100 units I think we've seen like 1-2 go bad... which is exceptional in our UK boating conditions.