The proper way to do it, is put some nut and bolts in it and shake it about. Air clean the inside of the tank using compressed air. Then put the liquid tank liner in. The problem with welding it back together, is it will probably explode! I did that once on a boat tank in a marine repair facility. The rust absorbs the hydrocarbons, then expels them when they get warm. This was a 20 gallon tank and it blew up in my face. I was not injured. The tank was destroyed. Video below shows the procedure.


https://www.youtube.com/watch?v=Sf2veir5VHU