WoodNet Forums: Bookcases over uneven surface: Hiding a seam?

In an old, new-to-us house in Melbourne, our living room is 19' X 25'.

Long ago it was two rooms; one with terrazzo floor and one with concrete.
The concrete section was a carport about 30 years ago. Concrete was poured to nearly level with the terrazzo and the two made one.
Not a great job but:
The concrete section is level. The terrazzo is level.
The concrete is slightly higher than the terrazzo and we plan to tile the concrete section.

The 19' seam between the two is variably uneven EXCEPT in the center of the room. The variation is between 1/4" and 1/2".
The most uneven areas are primarily at both ends of the 19' seam where it appears there was a concrete wall section removed from over the terrazzo.

In a discussion with the boss, I originally suggested we build two 6' wide bookcases over the uneven sections thus visually dividing the room into two areas: one roughly 19' X 13', the other 19' X 12'.
The space would have a 6' wide (roughly) walk thru between the bookcases.
I'll build a threshold to cover the visible seam in between the bookcases.

Tonight, she suggested we build two very short bases as if for a wall along the uneven sections and then place custom bookcases on the bases. Her thought process is it'll be easier for her to start the tile work and I can see her point.

What's the best way to build these bases such that they're level?
I thought I could build a 2" X 4" frame the appropriate length and width, set it in place with a couple of Red anchors, scribe the base, then copy the scribe line with a bandsaw.

The bookcases would be roughly 6' wide by 12"-14" deep.

Another option I suggested is to lay a form 19' wide and straight across the entire floor on the terrazzo side roughly 10" from the seam, pour self-leveling concrete over the terrazzo to even it up to the concrete. When it finishes curing, I'd remove the form, and we'd tile up to the form line. Then, I'd build a simple wood transition trim piece.

Am I way off base? Is there an easier way? How would a carpenter accomplish this to create a level base for these eventual bookcases knowing the bases will be in a highly visible area when first entering the house?

Thanks for sharing your experience.

--------------------
Gary

Liberty, Self-Reliance, Self-Responsibility
Say what you'll do and do what you say.
ServicePen 2013