I found a suitable right-angle 50-pin header for the SCSI card while wandering around in the Fry's in Palo Alto earlier this week. This evening, I managed to extract the straight header without damaging the board, and installed the new connector. The board fits in the backplane just fine now... and with a power cable swiped from a dead ATX supply and a suitable SCSI cable and drive from the pile, the card still seems to work.

I need to find or make up another cable for the silly MMJ serial connectors, and then I'll be able to try loading ancient Unix...