1 ton Rear Leaf Spring Replacement, hidden nut on front bolt ?

[F85Cutlass]

Hey everybody. I'm currently working on rear leaf spring replacement on my 89' V3500. Don't know. Going to loosen the nut on the front springs to frame bolt, and the nut which faces twards the inside of the frame is obstructed by a brace/bracket that is riveted to the frame. Wondering how many others have come across this and how they tackled it?

I'd like to avoid removing the rivets if possible, unfortunately the opening through which I can see the nut seems too small to even get an open ended wrench on though I have not yet tried.

Strangely enough, my factory shop manual does not mention this piece in the procedure.