sterbejj - In the first place, a builder can deviate from the plans. In the second place, it is prudent not to deviate from the plans unless you not only think you know, but actually do know, the what and the why.
The carriage bolt is common to other Glen-L sail boat builds. The carriage bolt is much stronger than screws. In fact the carriage bolt is often reinforced against axial rotation using screws distant from the axis of the joint fulcrum produced by the carriage bolt.
You must agree, the carriage bolt forms a very strong frame structure whose strength could not be created using any other method. Glues like rigid structures and work well under most types of stress. Screws in the same way are great for certain types of stress. Consider all the glued joints and screwed joints you have seen broken and guess the direction of the force. Now, think if those joints had been through bolted with a good 1/2" carriage bolt, what kind of force would be needed to break that joint.
I suggest using the carriage bolt and not lag screws or some other self tapping fastener mainly because it is specific to that one frame and that should be indicative of some need for it to be there and not elsewhere. Other than the pressure of the water on the hull, what keeps the hull from spreading should several tons of wave inspire it to do so?