Stuart; my concern would be water ingression from the space that was left un-sided. If in fact it's nicely sealed by the original construction. Why is it so deep? Is there strapping under the Aluminum siding, on top of the original construction?
That's actually a good thing; that's the same as a modern 'rainscreen' (mandatory now).
I think I'd just go with 'BlueSkin' then 2x2 vertical strapping and whatever extra is required to come up flush with the top of the siding, then shutters to overlap the siding slightly.
Again, I'm curious as to why the siding construct is so deep?
Is there some type of panel insulation or something backing up the siding?