Stretching another 2-3 feet for a 30 footer is not a big deal and believe it or not, Royal Lowell probably have done that many times with his wooden designs (usually during the build process, not after, tho). Stretching boats isn't common with fiberglass but it was with wood. If you look closely at old designs available for use, the designer will comment it is fine to stretch a hull up to one station mold. Same comments if want a shorter hull. One station mold is about 2-4 feet for a 30 footer if I remember the wooden boat building system correctly. So 2.5 feet is a minor stretch. And what's more... take a look at new lobster boats... you will see many 45 footers being offered in 46, 48, and 50 foot lengths. It's the same boat but stretched! If the beam is the same, it is literally the same boat. Stretches occur in two ways - a longer stern which is the easy way for a minor stretch. For larger stretches, the hull is cut in the middle or third then stretched (filled in, actually). That said, stretching or shortening a hull too much is not good. So I can understand your reservations.