Here's what the current "instrument cluster" looks like on the dash of the car, seen roughly from where my eyes are when driving:

The most natural thing is definitely to take over the recessed section, filling it with a new set of display elements.  The size of the area is slightly awkward though... approximately 300mm wide and 85mm tall.  That would ALMOST be perfect for one common automotive display form factor, a wide 12.3" diagonal display.  Unfortunately, the only ones I have been able to find in a usable form are around $300 which is more than I want to spend.

Using two displays side-by-side is a good option also, and I'm leaning toward that.  In particular, there are other things to consider that make such an option attractive:

• I am going to need to hook up a large number of digital and analog "sensor" channels to the systems of the car, which means a bunch of custom interface electronics and a microcontroller to read the values.  Perhaps that same microcontroller can drive one of the displays...
• I want to include complex software like mapping and internet access to the new instruments (at least eventually).  Doing that will likely be a LOT easier on a standard platform such as Android or Windows.  And adapting a strange oddball display may not be trivial... It would be much easier to just use an Android tablet or phone as one of the displays...