I am going to use a deep sump pan for my stock gas tank. Can this be done to a used tank or only a new tank? Don't want it to blow up when welding etc. Thanks.

If it's been empty and sitting open for a day or so it'll be OK.

I drained mine, rinsed it out w/ water, and let it sit outside for about 24 hours before we welded the sump on it.

dry solder. don't weld.
welding will open it to rust as well.depdending on the fuel you're using.

Empty tank and blow air through it for a while...
I prefer soldering it on so it doesnt worp

I would remove the pickup and sender as well as the filler tube and use a vacume cleaner that you can blow air from through the tank when applying heat,(don't try and suck air out of the tank with the vacume cleaner ) that or use nitrogen(not NOS ) or carbon monoxide out of a fire extinguisher as a filler gas to displaced the oxygen while working on the tank with heat

Great, thanks guys!

After washing it a few times and letting it dry in the sun for two days I took a piece of flex hose and ran it from my pit bike exhaust into the tank and let it idle for 1/2 an hour. Probably overkill but better safe than sorry.