Mopar used to make a tester box for verifying the gauges, works on all the gauges, temp, pressure, fuel but not the ammeter.

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

Not sure where that 300 ohms claim comes from, not true for a 74 and you have to match the sender to the gauge, not the engine