Sam Harper's site is what gave me the bow building bug! I must have 5 copies of those 10 or 12 pages from his site scattered around my apartment.
Now, as far as strings are concerned, I have a suggestion. I would suggest you get your hands on a spool of dacron b-50. I managed to score a 1/4 pound spool for 6 bucks, and it looks like it will make bow strings for the rest of my life and have some left over. I priced bowstrings, and this spool was cheaper than any prices I found online.
One reason I suggest this is, you want a longer string to start tillering with. You'd then use your shorter normal length string when you get to the point where you're ready to string it at "full brace".
Now that I've said all that, let me put a tape measure on my string: 60 and 3/4 ", or 60.75 inches.
I also started with the same size red oak board, 1" by 2" by 72" dimensional, which means actual measurements of 3/4" by 1 1/2" by 72". I cut my notches in 1 inch from the end. My brace is 6 and 1/2 inches, measuring from the belly of my handle to the string.
I don't remember if my string stretched at all.. but I think it's very likely that your string would work just find.
I would suggest that you start tillering with a longer piece of cord of some kind, maybe some strong rope you might have sitting around?
The very last thing I would suggest, is that you do a google search for "how to build an american flat bow", you should come across some vintage looking plans that are almost identical to Sam Harpers'. They did have some other info in there that helped with the build process a LOT.
One thing Sam suggests is using that drywall tape, 3 layers of it. That stuff is fiberglass. It works, but on my second bow, I did linen instead. That ended up looking a lot better, and was 1 tiny touch closer to "primitive'.
Hope this helps.
Riley, Saint Paul, MN