Components arrived from Digikey this morning! Not having the final boards, I printed off a paper copy. And found my first mistake.
Apparently SOIC packages come in three different widths. Updated to reflect the SOIC-8M size.
Lesson learned: double check packages before ordering boards. I didn't catch this in time for Seeed to swap out the order, so I've sent another order in. At $10USD for ten boards, it won't break the bank. The old ones will be usable as just plain proto boards anyhow.