Neal -
You're likely familiar with the voltage limiter built into the cluster already, as there's been a lot of discussion about it over the years. I suspect the test box was either a rheostat (variable resistor) or a multi position switch with a few different resistors to set the gauges at different points in the scale. You could do the same thing with a couple toggle switches and resistors, or an old head light switch.
The basic circuit is: 12V from key -> limiter -> gauge -> sender -> ground.
Some cars have the limiter built into one of the gauges, some have a little module that plugs in to the cluster.
The limiter is the first thing you need to check, because if it's bad, you might burn up your gauges.
With no senders connected, put 12V to the power wire on the cluster, and ground the cluster. Then use a meter to check the voltage on the sender terminal of one of the gauges. You should see it pulsing between 12V and 0V. If it doesn't pulse and is always 0V, either the gauge or the limiter is dead (check another gauge to help rule things out). If it is constant 12V and never pulses, the limiter is bad and should be replaced or you'll burn up gauges.
If you did one of the limiter replacement mods, you might see a constant 5V here, which is okay.
After you've done this check and have a known good limiter, you can test the gauges by simulating a sender with a resistor between the sender terminal on the gauge and ground. Resistance values to use here are in the 10-100 ohm range. I forget exactly, but I know Gary put my measurements of a fuel sender on the tech page at one time.
Remember also that these gauges have to warm up/cool down to move the needle, so they won't respond instantly like a modern automobile when you're testing them.
Dave Casey
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