Oil and gas gauge 64 Dodge

Before I yank out my dash cluster for a 3rd time, hopefully someone can give me an idea whats going on.
I have a 64 Dodge Polara dash with gauges and clock. I took the entire unit and gas sender to a vendor I found in Hemmings. I had the amp gauge replaced with a volt meter. I bought all new sending units oil and water, I cleaned the wiring harness at the bulkhead (new engine harness) and oem dash. The dash harness was a little dirty but crc really cleaned up stuff. I go to reassemble, I dielectric greased the bulkhead connection.
I put everything back together per his instructions and low and behold no gauges. Long story is he had me hook up amp gauge wires red to positive on voltmeter and black wire to negative post on gauge. Needless to say nothing worked other than clock and volt gauge. I then hooked red and black wire to positive post on voltmeter and grounded the gauge. Now I have oil gas and water. Well gas and water. Found out the pin for oil gauge on the back of the circuit board was loose. I take out dash n drive 2.50 hrs to have him fix pin and recalibrate gas gauge. Full tank of fuel only showed less than 1/2 tank uuurgh.
I thought all was well....wrong. new quartz clock doesnt work, it has power. Tonight I while in the car I turned key to run gas gauge starts to climb, but its only between 1/2 &3/4 and I start the car and I have no oil gauge reading, and the gas gauge is slowly dropping to empty.
After about 30 seconds or so, the gas and the oil gauges start working, how accurate I cannot say because now I wonder. I have good grounds on cluster / engine.
He put a new voltage limiter on back of dash one that has led light on it.
So I'm going to take dash out again. But before I do, what if any tests can I do to trouble shoot it myself?
Id like to be able have a better idea so when i take it back I can tell him to check this /that rather than I tell him whats going on and I fly blind and go through the hassle of putting it back together only to have to take it apart again. There are other things that were wrong but I've rambled enough. If I didnt spend 1k on this I'd bite the bullet. I really loathe some of these "so called" experts, I just wish my electrical skillsets were better.
Thanks,
Steve

I don't blame you one bit, for being irritated! For that kind of $, should have gotten a trouble free item.
But, since your going to pull it, anyway, might be worth your time to test each gauge yourself. They either work, or they don't. Example is your gas gauge...on the bench, it should (slowly) go to full. If it does, means either a wire issue, or the sender itself, that can be adjusted, if it ohms correct.
There's plenty of info, on how to test each type of gauge, to see if the problem is there, or something in the wiring.

Good luck on it!

start with the basics make sure there is good power going to the cluster including a good ground. Also that your voltage limiter is not intermittent check for output should be around 5.5 volts. If you have good connections at the cluster and proper voltage check your circuit temp and oil for a steady ohm reading once the car has run and stabilized and if not follow the wiring to each junction to see if there is bad connection. It is also good to know what the range is on your sending units and ask the guy repairing your cluster to vary the ohms showing proper gage operation that way you know for sure it's working correctly on his bench.