If the fuel gauge didn't read when you grounded the end at the tank you need to try grounding the input side of that gauge on the cluster to see if the wiring is the problem
I'm not sure on what the stock Mopar oil pressure gauge reads, ohms or mini volts
Cab..
Oil pressure, water temp and fuel gauges are all 5V DC movements...
As the gauge itself can be tested with a couple of D cells in series..
Each respective sending sensor is simply a variable resistor. Though some early 70s' A bodies have the voltage limiter built
into the fuel gauge.
Just my $0.02...
