Thanks for the input everyone. I have my answer, I guess I'm going to spend some time drilling and boring.
To answer the questions about what I didn't include before: the chunk of material I have is steel, I believe, I think probably part of the overarm from whichever horizontal mill this M head was attached to at some point in the past. My lathe is a WM210 lathe (9"x30"), and I don't currently have any drills larger than 3/4", even if my lathe could power a 2+" bit. I'll see what I can come up with, obviously I don't want to spend hours running boring passes if I can just drill it instead.