Adding a sump in tank

The safest method... Have someone else do it.

Honestly, if you really want to weld a sump on your tank yourself, I'd pick up a new one to start with. They are pretty cheap, I've seen several places where you can get one for well under $200 and then you don't have to worry about the possibility of explosion.

One side note, I've heard that the tank's coating, which I think is galvanized, can create toxic gases when you're welding on it. I'd be sure that where you're doing this is very well ventilated, at the least.