The drive was too small.
Followed the iFixit directions and swap of new drive for old was a snap — except for the very end. Felt resistance putting in one of the screws holding the bezel, but didn't pay it any attention. After I put the whole thing together, it wouldn't start. Took it apart twice before I discovered that the wire for the screen needed to be squeezed between the speaker and another part, otherwise it ends up directly under the screw I felt resistance on when I was putting the bezel on, and shorting out the machine. Once rerouted, the iMac started without a problem.
Pay attention to the details.