You shouldn't hhave to do any welding. It's not going to create a seal, stuffer plates just fill volume behind the impeller so you don't get a lot of churning water in that area. Anyway, you need clearance for the impeller nut.
I don't want to dissuade you from having your fun, but in practice, it's hard to beat a simple plate to fill the gap. And it weighs about 3lbs less.