Thanks!
On to a bit more progress. Nothing exciting or performance related.
Originally this car was equipped with rear heaters. Cores/fans mounted under the rear seats that blew through vents that were just under the leading edge of the seats.
I decided that instead of having just rear heat why not heat and A/C. Up front with be a VA Gen IV unit with fresh air/recirc door (no pics of that yet). Rear is another VA unit designed to be trunk mount.
I removed the original air return ducting from the unit (which curved up and forward) so I could build my own through a duct in the rear shelf.
So, as far as the pics go, the center large duct is air inlet (this system is purely recirculating) and the smaller outboard rectangular holes will be the top vents. There will also be vents under the seats as pictured.
I covered the vents on the rear shelf by a grill that matches those on the lower door panels. It sits in a steel frame that allows it to be flush mounted.