O D bell housing
It was kind of difficult to track down that info.
I kept reading posts about "simply" milling the retainer, but looking at mine, I knew that wasn't going to work.
Finally, I found a site that explained the multiple bolt circles and included all of them, not just the common ones.
If I had been able to do that, it would have saved me a ton of time.
Finding a small block 5.125 bell was not super easy, but fortunately they're not in demand, and therefore not terribly expensive.