The main problem is the illumination and screen ratio, you need a long, narrow display.
I think OLED will overcome all the ordinary instruments.
I have already made it a couple of years ago. I used an Olivetti quaderno with DIY backlight in my fiat126p. Tacho and speed with a Hall and a PIC16f876a.
Then I made a similar thing in my Seicento. A 7" lcd TV was applied in the middle and I displayed the speed, distance, accu voltage, rpm, temperatures, etc. It also served as the display of media center. After a while I did not watching the original instruments at all...
I STRONGLY believe, the central (and upper) position for instruments are better.
Perhaps you do not have to move your eyes up-down, refocusing, etc, just a sideway sweep.
Now, in my LagunaII the instruments are better, but I missing my good old lcd TV.
I am planning to fit a small, four-digit led display at the top of the dashboard and I displaying a mirrored speed, and it will reflect at the appropriate position of the windscreen (semi-HUD