Mine was welded and it still leaked before I got it.
Buddy at NAPA told me they have some good metal epoxies that will work.
I drained mine and put in water and used a sealant. Ran it until it was hot a couple of times and then drained it and refilled with anti-freeze and it seem okay so far.
I would try to seal it from both inside and outside before I went the welding route. I am also cheap.
