As has been said- the fumes in a confined space are the dangerous part. Filling with water removes most of the space that fumes can accumulate. Filling the remainder with an inert gas like CO2 or MIG gas, etc. (some people even use exhaust from a vehicle) will give even more of a safety factor.

Also, I don't know about that older tank, but most tanks these days are galvanized, and welding won't work on those. Soldering does though.

I have a friend that has welded a few tanks before....he runs exhaust fumes into the tank while he's welding it. the idea is to eliminate the oxygen, or at least replace the air in the tank with something that will not burn, or allow anything else to burn.

I used to have a link to an Army welding training manual that was on the internet. The above method was what the manual recommended..

Personally I feel safer filling it with water to where the water is just out of reach of where I'm going to weld or cut it...

For what it is worth. You do not need to weld. I have fixed lots of rotted tanks with 2 part epoxy. Some have been so bad I had to imbed fiberglass cloth into the mush to cover big holes. First clean all the crap off the outside of the tank and pull the fuel line tank fixture. Probably is totally crud anyway. Then check the inside for particles of rust or whatever may have got in there over time. Clean it out with a soapy wash and a good rinse. Once done, immediately get it dry as a bone. Take a scraper, a wire brush or whatever to get the outside cleaned and free from crud, including rust. Then get some phosphoric acid or phosphoric acid based cleaner and brush the whole outside of the tank with it, Let it sit. The steel will turn black. That is iron phosphate and a great primer. For the rusted areas, pinholes, etc., clean them down to bare and mix up the 2 part epoxy paste. Smear it on with a spatula or a paint stirrer. Use the fiberglass if necessary. About an eigth of an inch thick. Let it cure overnight. If in a hurry, a hair drier will do the job. When cured, hit it with the phosphoric where the metal is bare. Wash off and dry. The paint with rustoleum or equivalent. Once the epoxy is set up, you will not have a gas reaction with it. The rest of the truck will rot away before the tank repair goes.

Additional trick. Many tank straps are steel on a steel tank. Get an old bike tube, goober it up with grease and shove the strap through it. Now you will have a greased up, impervious to water and salt strap with a rubber outer layer that will not rub and cause the tank to rot....

Lotta work but you can save a really bad tank with minimal bucks....

I had the tank for my '68 Cutlass "dipped" in some plastic coating for about $75. That was almost 20 years ago and it's still not leaking. A local radiator shop did it as a sideline business.