You better be a good code welder to make the frame an air tank - the pressure needed to run airbags will blow the truck apart if it fails. Most air tanks sold are not certified either - it's a big gamble but worse if the air tank frame bends in a collision, it will have the air pressure to cause it to fail. I would not trust any 20+ year old frame to make a tank out of - too much stress over the years fatigues the metal.