In reality the ratio that has to exist for it to explode has pretty small limits. I have welded on them doing nothing but leaving them in sun for a few hours. Thats why the guys running exhaust through them and people that just walk up and start cutting get a away with it.
exhaust is NOT the way to do it. Exhaust contains unburned hydrocarbons and carbon monoxide wich can both be explosive in the right ratios with air. You could actually be making it worse, there have been documented cases of tanks blowing from using exhaust.
Be aware that you can excedd the fuel to air ratio and it will not explode either.
Yeller has one of the safest methods, I have used it several times. Leave it in the sun and let a shop vac blow through it for a while. You can actually test the air leaving with your torch. if its flamable it still wont explode because the tank is pressurized and the flame can not go backwards into the tank but chances are it wont anyway. By blowing air through it for a while you evaporate any gas in the tank AND you make the fuel aire ratio too low to explode.
You can actually strip it with a liquid dish washing or a harsh laudry soap and water before you blow air through it, or you can flood it with nitrogen and weld it with the nitrogen flowing. Or even Carbon Dioxide. But to be safe just make sure its dry before you start and test it by blowing air through it. I have a friend that put about a quart of acetone in the tank and sloshes it arround then pours it out to strip out the gas then lets it dry a while with a shop vac running on it.
These things do blow up from time to time but a lot of people seem to be able to do anything to them and never have a problem. My father was a tank mechanic in the army and a welder until he retired. He told me they actually blew a Jeep tank up in the motorpool. They got the bright idea to run exhaust through it because thier sergeat told them it would work. They blew the tank with three men standing around it. All that happened was the tank slipt and made a loud noise that it took their ears a while to recover. no one was hurt even though they were standing there with it when it blew.
Just because some ones method works doesnt mean its actually safe. Just make sure there is no liquid in the tank and then flood it with air for a while, or put in something inert like, nitrogen or carbon dioxide .
Back in the late 70s when Carter was president and everybody was carrying around a lot of extra gas just in case they couldnt get it when they needed it, I saw a guy weld on one of those rectangular tanks people put under their and behind thier tool boxes to carry diesel but he had filled it with gasoline and a seam was leaking that didnt leak with diesel (or at least not enough to care). He is still alive today because the fuel never saw any air as the weld was below the liquid level. Pretty sue no one would recommend that.
If you had to ask, Its actully best that you dont do it, ultimately there it will probably not blow up no matter what you do. But it can and does happen, so you want to do everything in your power to make sure it doesnt happen to you.
If its just a return I would just do it at the filler neck. You can snake a return line all the way down into the tank. The only reason I would go to all the trouble to weld on one would be to cut an opening to put an in tank pump in the tank, and even then unless you bought a good 23 gallon tank and later decided to fuel inject I would just use the oportunity to get a bigger tank that was ready for an in tank pump and return line.
I fuel inject a 55 chevy back in teh late 80s. I used an external pump but I pulled the fuel sending unit and drill a hole in its cap and soldered the it up and ran the fuel line right down next to the pickup tube. Simple and safe.
I strongly recommend you leave it be.
