Q. is the ellipsis in a block quote centered?

Well, the first thing I would ask is if maybe this would be best handled by paraphrasing. If it is information spanning paragraphs, a direct quote may not be the way to go. This is not a hard and fast rule; it just is something to consider. However, if using the direct quote is the best way to convey that information, then that is fine.

I didn't find any specific confirmation, but it seems that centering the ellipses in this case is best. I did find confirmation that it is correct to use centered ellipses when a line of poetry is omitted when quoting several lines. 

I linked to the example I found below. It is in Purdue Owl's MLA guide. Scroll to the bottom until you see the headings for Long Quotations and for Adding or Omitting Words in Quotations. You'll find instructions and examples, including the one for omitting a line of poetry.

The guide gives info on different types of references, in-text citations, general MLA format, sample papers, etc. The pagele of contents is on the left. Just select the section you need to see. Hope this helps!