MDF is great stuff but I don't think the application you are talking about would be the best use of it. I assume you're using an internal strong back. Flake board (oriented strand, OSB), or 1/2 plywood might be better materials to make up a box beam. I've never used an internal strong back, but no matter what you use I would think it will be fairly strong along the 4" dimension and correspondingly weaker across the narrower side. However I know that a lot of boats have been built using such a strong back. Alternatives might be 3X6, 4X4, etc., but I know you'll probably run out of room somewhere along the line. One thing to think about is that as long as your strongback is strong and stiff (immovable)...even if it does have a curve...you can still line up the forms to be straight and level. Hopefull those who have actual experience with this type of strongback will put in their two cents.