There was a great photo posted here (last week) of a 8B on a 990 in a discussion of the 448 hoe.
From what I could see, the mounting brackets a a bit different from my 8A (on a 970) but not much.
And based upon what I could see, I wouldn't expect that you'd have too much difficulty mounting the 8A to your 990.
You will also need the Power Beyond hydraulic connections from the 1070 as well....
AKfish