I've moved the above posts to the original Oryx thread, so please post any others there. Thanks.]

I don't think there's a hard and fast rule about this. As Pete says, he builds a mast first, and then builds a tabernacle to suit it. I would guess that he tapered Speedwell's tabernacle to reduce its bulk below, and to get it to land on the top of the keel neatly. Such thick wood would have to be steamed to bend it, I think, though maybe a powerful clamp is enough to pull the sides in. There's nothing to pull against, to pull the back into a curve. I'm not sure that scarphing is a good idea, I would rather have the sides and back running straight through. I've only made two small wooden tabernacles, for a dinghy and a dayboat, but from that experience, I would say to keep the sides parallel unless there's a good reason not to.