Done! Details to follow.
Edited to add:
I did bar the engine over a bit, and that seemed to help.
I removed the pump, got out my thread chasers and chased the bolts and the holes in the block. That made some difference, I think.
I could tell from looking at the engine on the stand that the pump arm needed to go under the cam, so I made a point of pointing the arm down as I put it in the hole. I'm optimistic that it's underneath the cam.
I finally took some 3/8NC allthread and made a stud, threaded that into the back bolt hole, and slide the pump into place on it. I was able to get a nut started on it and used that to suck it down into position, then got the front bolt on. Finally I tightened them both down.