Around here the outside of a building usually gets finished before the inside is rocked. Easy matter to locate a wall pack so it's not on a rib. We leave a tail hanging out from the inside to be connected later.
You say you used 4x4 boxes. 1900's with rings? I suppose you could use a raised ring high enough to be flush with the top of the rib then cut something the size of the fixture to fill behind it. I can't think of what to use though, I would want it to last as long as the siding.
Any way you look at this you have a mess, the fixtures should have been located so as not to land on a rib. I don't know what you could have done though, sounds like an ass backwards method of construction caused this.
-Hal
