The 38 CB is actually 38'9" in length, so to get to 42' would require 3'3" additional length. So I would think they could get away with not extending the running gear aft. I have no knowledge of Stewart's plans, that's just my opinion.
I have seen many 37' Wegley hulls (west coast) extended 3'-5' and they all perform just fine. Wesmac routinely stretches their 42' to a 46', and I don't see them extending the running gear, so it must work good. Even the 38 Wesmac (actually 38'8" in length) can and has been shortened to a 36' hull. So the running gear on a 38' Wesmac is actually where it would typically be on a 36'-37' hull, and the 38 Wesmac is a really responsive boat at the dock. So in my opinion, I don't think it would be necessary to extend the running gear if you are adding approx. 3'3" to a hull.
I also think the 38 CB runs at a good attitude, but the additional length will allow it to carry weight better and more efficiently.