Fuel gauge fluctuates

Well the book will tell you between two to three hours. But that is with all new fasteners and using a lift and having all the proper tools right there.

In the real world you can expect about four to five hours. Make sure you have the fuel line disconnect tools. If the mileage is correct and you still have the OEM pump, plan on replacing the entire assembly of pump, level sender and replacing the pump connector. This is because a good level sender is over half the price of a GM pump.

But I will let you in on a secret, It is usually a lot easier to do the pump replacement by unbolting the P/U box and lifting it up for access. No wrestling with the tank or bending or breaking fuel lines, plus you have it all right in front of you at easy seat height. Lets you clean it up and remove the parts without needing to feel your way around.
Works great for regular or extended cab versions. Just spray the bed bolts down good with some penetrating oil like PBlaster or Kroil. Remove them, lift up the bed and either prop it up or slide it back. The two part video is pretty good, there are many others if you want to see it done other ways.

https://www.youtube.com/watch?v=aEaJ6pxWEcI

This is common with the S series trucks. The way to fix it is to replace the entire fuel pump assembly, which costs more than $220.00 for a cheap aftermarket pump, which will likely only last a year or two. Personally, I would not bother with it.

Lowering the tank does not take long. First, depressurize the fuel line by taking out the fuel pump fuse or relay while the engine is idling. Then unhook the fuel line where it connects to the fuel filter (should be on the driver's side, under the truck, along the inside of the frame rail). There are two straps wrapped around the tank that are bolted on one end. You will have to look around the tank and unhook the various hoses, unhook the fuel line, and then undo those bolts.

If there is much gas in the tank (more than a quarter tank) I would suggest draining that fuel before putting the tank back up. The weight of it makes it a real challenge to put the tank up and put the straps in place, and bolt them up, all while laying on the ground under the truck.

I have heard of people taking the bed off instead, and leaving the tank bolted into place. Apparently it is easier to do that if you have a helper. Also, I have heard of people cutting a hole in the bed to create a service hole of sorts.

Hope this information helps. Please let us know if you need anything else.

Apparently Steve and I were writing at the same time. In general, if you can avoid dropping the tank, do it. I have done it a couple of times on my Blazer and it is not fun. I have a system, but it is still a feat of strength and stamina getting it back up and bolted in.