: 1) Why does he spec the rib thickness at 3/8", when many other
: designs use 1/4"? I realize he has me thinning the bend
: areas to 5/16" but it still seems like overkill. How about
: 5/16", thinned to 1/4" at the bends?

Most (practically all) people use 1/4 for the entire rib. This is one of Cunningham's ideas that he published but has no reason for doing. You will find more.

: 2) I'm using White Cedar for the gunwales, keelson, chines. I've
: heard it bends nicely, and wondered if I could use it for ribs
: also? IOTW, are hardwood ribs really necessary?

For most people, oak and ash are the most accessible woods. If I had unlimited selection I would probably try some of the cedars that only people in the Pac NW have available. I live in Vermont, however, so oak it it. If you use white cedar, however, you will probably want to make them wider as they are not as strong as hardwood (which might negate any weight savings). Morris uses white cedar and makes his ribs 1.5inches wide.