How to step-by-step remove the gas tank?

The older trucks are a bit easier to work on but if the tank has fuel in it, you would be better off running it as low as possible. As for repairing it or replacing it, a new tank like the Spectra Fuel Tank GM16A that runs around $120.00 online. The proper repair would be to drop the tank, steam clean it to remove any fumes and then braze up the leaking spots and galvanize it. That repair will run at least twice the cost of the new tank, if you can find anyone still in business. The reason it's leaking again is likely due to the alcohol in the fuel, it is a powerful solvent and will loosen and dissolve most glues and epoxy type products that are in the common repair kits. I just want you to know that ahead of time.
As for removal, the easiest way is to remove the truck bed first. That lets you get to the top of the tank and the fuel lines easier and to the top of the tank straps, so you don't have to fight with them from under the truck. The other option is to get the truck up in the air on ramps or jacking it up and putting it on jack stands. Once elevated you reach over the top of the tank and release the clamps that hold the fuel lines and then reach up and remove the clamp on the fill neck hose. Also, spray some form of lubricant on the tank strap threads and nuts. I like to use a mirror to be sure all the lines are undone. Then use a small bungie cord to tie them out of the way. Now get the wrench that fits the tank strap nuts and break them free and then use a ratchet to back them off a bit. Now take a floor jack or similar and raise it up to support the tank, remove the strap nuts and release them, then lower the tank down. Now you wrestle it around to either do the repair or replace it. Now one thing, they offered a 13 gallon and a 20 gallon tank in those, the replacement tanks are now the 20-gallon units. As such if you have the 13-gallon tank you need to buy the tank straps and the sending unit for the 20-gallon tank. I would also replace the fuel strainer in the tank and being an 83 I would replace the rubber line segments as well.

If it's the 2.8 it should have the 20-gallon tank. The 13 was supposed to be used with the 2.5 4 cylinders but if it was a fleet order it could have a 13. Never seen one in the wild but you want to make sure first.

You are welcome.