Elliot,
First ditto everything Rehd said and then ditto all the stuff you already have figured out. Your right, this is the foundation of your boat.... don't pour sand

The biggest thing I can recommend is take your time. Don't just build it and move on. Stand back and eye ball it, tweak your form endlessly. The trick to getting everything straight on your first boat is to work it a while, then despite your enthusiasm, force yourself to walk away. Go to the fridge for a sandwich Come back and work it some more. Down 20 Orio cookies work on it some more. Take a nap come back to it. etc Keep puttering on it until you have it perfect. or the snacks have added 20 pounds to your own midrifts. I keep a barstool on both ends of my boats during this process, just so I can stare at the boat and look for any imperfections in alignment. When you find something that could be tighter, work it, but work it knowing that anything you do is working all the other sanity lines out of wack. Its good to have more then one set of hands an eyes as your pulling stuff into/out of alignment. Some one working close and someone looking from far and askew.

Here is a trick for string marking the beam. Measure both ends to find the middle. Run a string down the middle between two nails or adhered in place by duct tape. Then get some bright spray paint and lightly spray over it, all the way down the string. The line you get will be true and better then a snapped chalk line.

Also get your self an 8 inch speed square. IT will give you faster 90's on the stations in both directions. Some times I 'll set two or use hard wood blocks I know are squared perfect.

Finally, after you have it set in place for a first placement, start tweaking. Tweak and measure endlessly. Run strings on all the sanity lines that come with the Guillmot paste ups or if you did a lofted version mark those lines 6 to 8 per station so you can be sure your gettting it perfect in 3 dimensions. When you have that in place and it looks good take another good long stare, walk around that form. Keep lining those lines up with your eyes.

The place most folks get in trouble on internal strong back built boats is the stem and stern. It's a long run to keep straight. Putting shims along the measured outside line for the stations is a great first step.

My suggestion would be if its your first boat. When you think you have it perfect. Grab someone else with a keen eye to back stop you. Let them look it over in a detailed way. Work out the things they see. After you have it perfect to your eye and the eyes of your second. Commit yourself to not building for two more weeks, but instead spend 2 weeks with a string line. and a bar stool and a roll of duct tape. Work it only when your fresh and feeling detail oriented.

When I am setting up a form for the first time. I like to tape string along each sanity line. When I have all 8 in place I can see the outline of the boat and abnormalities will show up more clearly. You can do the same thing with duct tape. It will give you a cool read on the final shape of the boat and point out any problems. If after all of that effort you have all your strings doing the right things then and only then is it time to build.

You are very right the most important part of the project is what your doing now. Spend the time, lots of time, sweat the details and send us pictures of your success.

!RUSS