I'm at work and can't dig into the plans right now, but yes, the way to lengthen the boat is the same as shown for lengthening from 26' to 28' ... you space out all stations from amidships aft, e.g. from the widest point in the hull to the stern. At the lengths that you're talking about, the boat is getting fairly long for how wide it is, and aspect ratio impacts maneuverability (it goes down as the boat gets too long for it's width) and it impacts roll stability (rolls easier as the boat gets too long for it's length). Finally, as the boat gets longer (and I won't dig into the details right here), it's going to float higher on the water. Too high and the boat risks some roll-direction stability issues .... "chine walking" where it'll plane a little heeled to one side, then flop over and plane a little heeled to the other side. So.... What's the limit for length without also making the boat wider and heavier to compensate? I dunno ... I only modeled the boat to 28' long. I've had several design ideas presented to me, but frankly, a boat this size is a big job to redesign so I can't ... no time in my current schedule. The longest version that is working well is Kent Cannon's 29' 4" one, but he also tends to load pretty heavy and that helps compensate for extra length. I usually tell people that if they want to stretch the boat longer than I specify, that it's up to them and that it's their experiment, not mine (sorry if that's too frank - I tend to be straight forward and open). If you DO experiment with long lengths and have good results - then it benefits everyone else who might want to. Also, the GA is designed on the conservative side, with a deeper waterline than is required (by 1-1/2 to 2 inches) since that helps guarantee stability, so there is a certain amount of forgiveness built into the design. I did that specifically because amateur builders like to enjoy some flexibility with the design as they create the boat of their dreams. So ... you're now armed with more knowledge ... go forth and conquer!
Brian
