I am thinking of doing this in my Ford Anglia (with Mk2 Escort running gear). The speedo comes in via a cable from the gearbox, so I'm thinking of attaching that up to the remnants of an old fan with RPM monitoring. The rev counter uses from what I can tell electric pulses, maybe again I can hook that up to a fan counter. Fuel and temperature again are just electrical signals that tell the gauges how far to move, so just need an input from those.
If I can find a nice DOS (or small-footprint Linux) application that I can read these and display them on the screen, then I have a spare Epia 800 and 6.4" LCD which I could use to boot quickly when I turn the key to display all these details.
I was trying to set up a 2nd display on my Epia M10000, however my Xenarc VGA display wanted to take over the TV-Out bit and display that too, so no dual-monitors with that, unless I add a second display card (but needs to be a low-profile one...any ideas). If not, then I'll just set up a pair of Epia's in the car!
What "very old car" will you be putting it in?