The fuel and temperature are fairly easy to test.
You'll need a good source of 5 volts. With some work, an old computer power supply can be used (google "convert computer power supply to bench power supply for directions) or rig something up with your new voltage limiter.
Then you need some resistors or you can spend a few bucks on a cheap Decade box like I did. More info here:
c-3826 tester for gauges and sensors
For the switches, the first thing to do is a simple continuity check with a multimeter. That will give you a good idea if they work at all, but the real test is when you put some current through them. I'd rig up an old headlight and a car battery or something like that and try the switch that way. To test it further, you can do a voltage drop test across the switch while you switch the headlight on.
Nobody ever watches this but I persist in posting it because it's good info about voltage drop.
The ammeter is a tougher one to bench test, although it can be done. Honestly, doing a bypass under the hood will take a lot of the current load off the power wiring under the dash and through the bulkhead disconnect. It will make the ammeter pretty much useless though. Here is a good explanation of the hows and whys.
Some considerations about the charging and wiring upgrade and your worries about It's important to use a fusible link if you do that mod.
CBarge did a thread with better pics on the way it's done.
Underhood Ammeter Bypass Read the Charger link first though.
Here's how I rebuilt, one wire at a time, the wiring harness in a '53 Chrysler. Just a piece of plywood and some finishing nails. I've been factories where they build wiring harnesses for high end military electronics and this is the same way they do it.
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