It's possible with MIG, but my experience with it proved that I get a lot of tiny pores in my weld. I'm not a good mig welder, and much prefer tig for jobs like this. But if it leaks a little, I wouldn't worry. Smear some JB weld on it and forget about it. If you get a few drops per minute will be absorbed by the scuppers and nobody will ever notice. If you want to test it once you weld it up, you can pressure test it. You'll have to get creative to close the big end, and bolt a piece of wood with a rubber gasket across it to the small end. Use the water injection port to supply pressure. You don't have to pressurize it a ton, just enough to make some bubbles with soapy water. Use pencil to circle the pinholes.