fuel tank repair/ modifications question

going to install my new fuel pump assb.. so i need to cut a section out around the oem fuel pump installe area, to a larger hole, and at the bottom of the tank were those two sheet metal guides ( ? ) are located.. i can weld very well now.. but to make sure the welds have no little leaks,, should i cover the welds seams with a tin/lead solder?the tank has been empty for 3 years now,,,, i cannot smell any fuel.. plane to wash out with dishwashing ( dawn ) soap to make sure.. but i also plane to put dry ice chunks into the tank during repair