HOW DOES ONE REPLACE RHE FUEL LEVEL SENDING UNIT ON A 74 CHEVY PICKUP

You could use two resistors, Two different ohms specs for full and empty. Place resistor between proper wire colors in diagram. Turn on the key, If gage reacts properly to full and empty resistors, then I think the problem is the sending unit in gas tank. If not, the problem could be almost anywhere, wiring circuit--pcm--instrument cluster. I did see trouble codes relating to fuel gage sending unit. I hesitate to get into that? good luck

The fuel level sending unit inside the fuel tank is bad or the gauge is or there is a wiring problem This video is for a mercury vehicle , but all makes an models are the same . Repairing Fuel Gauge Problems 2004 Mercury Marquis EricTheCarGuy

How did you check your fuel gauge? The "Universal" fuel sending unit could be a problem.

if it worked for a short period of time after doing the pump/sending unit the check the connections going to the pump/sending unit they would have been moved around doing the pump

SOUND LIKE A SENDING UNIT PROBLEM! YOU MAY NEED TO REPLACE IT AT THE TANK!

fuel sending unit located on the fuel pump in gas tank

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/thomas_c50676e836a18949

sending unit on fuel pump is bad, the sending unit shorts out gauge will go to empty or full, never read right, turn on switch ,it goes to full or empty, ,check the pump,hope this helps,thanks robert b.

You have to drop the tank, on your fuel sending unit if the "float" that tells the gauge how full your tank it. These like to get holes, or jam up. So it is only going up half way, and your gauge only reads half full. So it could have a hole, it could be sticking, or a variety of things. Before you go doing all that, I'd check the ground wire off your unit, since it is working doesn't sound like a ground issue, but they can be tricky. Good luck.

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge