I doubt very little, it looks like the main function is to provide vertical support for the back half of the upper deck in the bow area. It probably also helps counter any torsion put on the bow,, like a wave hitting the bow on the corner at a 45 degree angle.....putting a twisting force on the hull I think that would put more stress on it than any fore aft stress. The seacast should be overkill for that too.
I was actually thinking of making it wider for workability while pouring the seacast.....its kind of like peanut butter, but I think I can manage with 3/4". I don't think I would go any thinner than that however.