UPDATE:

I couldn’t find a local distributor of Dayco, so I went with a Gates belt, one size narrower than the book calls for. Then I also made an adjustment to the bracketry; although it looked straight, when I put a straightedge across the pulleys the alternator was a little proud of the crank and pump. So I moved it back slightly and reassembled.

Well, Either I fixed the alignment, the narrower belt worked, or (most likely) it was just a freak deal from the get-go, because I made 3 full passes on Sunday with no issues whatsoever.

Car went a new best as well, 11.321 @ 117.89; but that’s for another thread.....