Well, car exhaust nowadays isn't that oxygen deprived. what kind of welding are you doing? Mig? Tig? Stick? Can you ventilate the tank well? I'd probably want to give it a detergent / water rinse only to eliminate having oil soot in the tank, if you can open a couple vents there won't be any explosion, maybe a little fire inside. Explosions can happen if its air starved and suddenly gets air. If you were flowing air into it, and it was completely drained, you'll get smoke but no kaboom. What are you doing for fittings? oil will make a weld really ugly and weak.
I'd strip the tank from the chassis, soap/water wash, bore your holes, figure your brackets, wirebrush all the paint from around the weld areas, soap/water scrub rinse again to clean it, dry it and then weld on the tabs/fittings
if needed, with the tank removed you can do everything with a flat weld. If you're worried about the smoke build up, you can dilute it by putting a shop vac exhaust into one of the ports and keeping it well vented. Though after a wash / rinse, I doubt you'll have much save for any tank coating. A good final wash will rinse out any soot or junk. Dry the tank, a little denatured
alcohol will help catch any moisture and make it dry out a bit faster.
-Jason