: Why do you suppose Cunningham uses a thicker, but narrower, rib?

I have no idea, but he's the only one who does that. It doesn't make any sense to me to use thicker wood and go through all the hassle of thinning it to make it bend easier. On top of that, you end up with weaker gunwales due to the larger mortises you have to cut in them.

: (3/8 x 7/8, thinned at bending areas to 5/16) He places a rib
: every 6" - is that standard?

No, though there isn't really any real standard for rib spacing. I've always calculated the spacing to give me evenly spaced ribs between 8" and 10" apart.

: I'm inclined to stick with the book's method, as its my first SOF;
: OTOH, I think your rib is lighter; OTOH, I already bought a very
: slick 5/16" router bit! Darn!

Thinner ribs are definitely lighter, but that's really not the issue. They're easier to make and you don't waste wood that ultimately serves no purpose in the finished boat.

: I'm sure in the end it won't
: matter, either way will make a perfectly good boat.

True.

: I tend to
: sweat the details when I'm new to something. I really want this
: one to be lightweight, however.....

Don't obsess over the weight. No matter how you build it, it's going to be much lighter than a commercial boat. Frankly, I've never used cedar for gunwales or stringers and I don't think I ever will, as it's not very strong and dents too easily. It's a excellent wood for paddles, but I prefer something stronger for frames, like white pine.

: Okay, the other thing he does differently is his skinning
: technique, using staples, and stitching along the sheer. Anyone
: care to comment on that? I've only seen the video on
: skinboats.org, as an alternative and, I assume, more typical
: method.

There are several effective methods for skinning and you can more or less pick the one that you think looks best.