1968 barracuda temp gauge and fuel level inop

Is that an oil pressure gauge, or an oil pressure light? An oil pressure light circuit has nothing to do with the circuit that feeds the temp and fuel gauges, so the operation of an oil pressure light is not of any use in troubleshooting this problem. An oil pressure gauge IS fed by the same circuit at the other 2, but most of these cars have oil pressure lights only.

BTW, you asked about testing the temp gauge sender; you can test that to see if it is good. Put your multimeter on the resistance (ohms) mode, and select the lowest scale (usually 200 or 400 ohms). Heat the sender up in the engine or in a pot of water with a thermometer to around 180 F. After the engine is warmed up normally, disconnect the wire from the temp sender and check the resistance from the sender's connection to the body of the temp sender; it should read in the range of 20-35 ohms; the exact number will vary depending on the exact temperature and the actual sender, but it should be around that range with a 180F thermostat and normal temps. I do this when driving; I just stop after the engine is warmed up, disconnect the wire and make a resistance reading on the spot.

When you tested the temp sender before and got .01 to .03, was that volts or ohms? An ohms reading while the wire is connected is meaningless. If that was volts, then something is wrong in the dash with the gauge or the voltage limiter in the dash. Read the voltage out of the wire to the temp sender with the wire removed from the sender, and the key in RUN; you should see the voltage jump up and down erratically from as low as near 0 volts to up near 8 or 10 volts typically; each meter reacts differently to this.

The voltage at this wire should be a pulsed 12v if the voltage limiter in the dash is good and the gauge is good; meters don't read this pulsed voltage well. It is actually easy to see by connecting a 6v, 1 watt lantern battery from the wire to ground; it will flash brightly about once per second. You can get such a bulb at Radio Shack: model 40 or model 1847. (Pick up some test leads with alligator clips while you are there....) You can run the same test at the wire to the fuel guage. If the bulb flashes brightly, then if you leave the bulb connected for 10-15 seconds in either circuit, then the associated guage will read on-scale if it is good.

Pete mentions the 27 ohm resistor (or 24 or 22); you can use that as a sensor substitute in either the fuel gauge circuit or the temp gauge circuit; either gauge will read around half way with a resistance in the mid-20 ohms range put in place of the sensor.