I finally dug out the door threshold over the weekend. It was a
standard 2" x 10" with a 1" x 2" nailed to it. It was nailed to the
bottom of the door jamb, but since most of the wood there was rotten,
the nails didn't present much of a problem. No reshaping of the new
piece of wood was needed, the slope was built into the concrete (new
piece will slide in from the inside when I'm done putting coats of
polyeurethane on it). Lowes had "appeareance grade" 2x10's that
looked good (and had squared edges) and the 8' piece fit into my car
(barely) and my table saw was all I needed to cut it the right length.

Interesting enough, since I've never worked with rotted wood before
was how little of it showed on the surface.