WillS hit the nail on the head!
Making a 75#-90# yew warbow out of that piece of crooked yew, is very ambitious to say the least. Not only that, but it is also your first bow ever. Did you think that the people making the bows during the hundred year's war were inexperienced novice bowyers just pooping out those warbows? They were highly skilled and experienced craftsmen! It is just not realistic for a first bow to become a warbow. You will even struggle staying above 50 pounds; trust me. Shooting it is yet another matter. If the only bow you have is a 80# warbow, you cannot learn the techniques. You need to crawl before you learn how to walk. Learn the techniques with a 40-45# bow, gain muscles with a 60-70# bow, and only then - after a few years of training- you can safely switch to an 80-90# warbow.
I hope you're realizing that your original plan (building two bows, one warbow, getting some proper wararrows and living the past in two months time) just isn't feasible. I don't want to talk you down, but you have to be realistic. Learn how to build and shoot a normal hunting/target weight bow. Learn how to work with wood and how to tiller. Your experience with boat building will certainly help, but a bow is a completely different, dynamic, creature. The boats have only taught you how to work with the tools. Not how to read grain, how to work with knots, how to bend wood, how to tiller, how to monitor draw weight etc.
Well, ok. Here's some more specific advice.
- A bowyer cannot just build two bows. Once you start, you're hooked
You'll find bow wood everywhere, and see new designs to try out all the time. So be aware of what you're getting yourself into. And warn the miss as well. You may have to make room in the shed for a wood rack, and may find some place in the house as well for keeping the finished bows.
- Drying wood outside or in a shed is possible, but only up to a certain point. Wood needs to be dry before you really start tillering it. Outside is generally too moist to reach an adequate equilibrium moisture content (EMC). Once the wood starts flexing for floor tillering, you need to put the wood indoors for at least a week or two. Also finished bows must not be stored in the shed.
- That 80" yew stave is not a bad piece of wood, but it will indeed not make a warbow. Did I read it correctly that it it 4" diameter? If so, you should be able to get two bows out of it. Considering it was a horizontal branch, one bow was facing the sky, the other was facing the floor. Don't use the sides of the branch to make a bow; only the upper and lower portion will make a bow.
- Start with the 60" stave for practice. If you're lucky you might make a nice kids bow out of it. But I would actually recommend you to buy a dry board of hardwood from a lumberyard so you can start working straight away. A board is much easier than a character branch of yew - by a mile.
- Make a bow first, before you worry about arrow spine. You need to know the bow's draw weight and draw length to find the correct arrow spine necessary. Only then you can order the shafts (and maybe arrowpoints too, although the specific arrow heads you're after are only period on warbows). "What is arrow spine?" is a really basic question that is easily answered through Google.
- As for dimensions: for the 60" stave, start with full length (obviously), 1¼" wide in the center 10", then tapering gradually to ½" nocks. Thickness 7/8" tapering to 5/8".
Whatever you do, keep us updated and post many pics, so we can guide you through the process