I've seen a quote several times - something to the effect that if you changed the design of his H-28 one tiny bit, then the birds would not continue to sing over the companionway - or something to that effect. Too sad, as I have a flushing head and no cedar bucket on board!
Amy, it's at the beginning of the same chapter as the gun in the cabin. He has already explained the advantage of the outside ballast over inside ballast, which makes the difference between safe groundings and breaking up, and between a clean bilge and a smelly mess. He also specifies bent ribs, not sawn (much greater strength), and wire shrouds and stays, not rope fiber (deadeyes, at least I think that is what deadeyes means, a bit before my time).
"If the H-28's design is only slightly changed, the whole balance may" (note the word 'may', less dogmatic than usual) "be thrown out. If you equip her with deadeyes, build her with sawn frames, or fill her virgin bilge with ballast, the birds will no longer carol over her, nor will the odors arising from the cabin make poetry, nor will you soul be fortified against a world of warlords, politicians, and fakers."
So, I think you will be fine. I'm sure he'd approve of fiberglass, he is always concerned with the least expensive and most efficient way to build, and he specified wood that resisted rotting (fiberglass is the best 'wood' for bending and rot resistance). No cedar bucket, well, that's just in with removable add ons really. He also mentions "As for the proportions of beam and draft, we cannot suit all unless we design a rubber boat." So I think he'd be 'flexible'