A custom cluster sounds like a great idea. I have toyed with it myself, but never to seriously as the only thing wrong with my cluster is the broken speedo cable.
A club member put an entire custom cluster setup in his FJ-40 that worked really well. For running your temp gauge you can should probably run new wires down to the sensor on the side of the block. Most after market temp gauges are designed to plug in, if you check the FSM you can probably even get the resistance range of the sensor in order to make sure it matches your new gauge.
For the Speedo, I would look at replacing the kit with a Universal Speedometer. Your buddy at the race shop will probably know about them. The FJ just presses a button, drives a mile, presses another button and it's calibrated. Great setup if your running non-stock tires and gears. If you want I can talk to him and get more info on the gauges for you.