[G.A.S.]

I dont understand where welding came in the picture. but dont be welding on an oil pan while it is on anything. You will find out what (internal combustion) is all about.
The engine definatlly needs to be cleaned so you can locate or isolate the problem.
Look higher up on the engine after it is clean. A valve cover gasket could be your culpret.
A non drip situation and that much oil being gone tells me your burning alot of oil, Can you see it smoke? If there is nothing on the ground in 5K, It cant be too bad. A little oil goes a long way.
Clean and inspect and go from there,