I had exactly the same problem with the GM tank and an amature patch over the filler hole. First fill up and woosh!
Mr. Fixit is spot on. Take it to a good radiator shop. That's what I did and it never leaked again....there. I did later develop a leak somewhere around the new filler neck in the tank. You might want to check that too. Like the other posts said, fix everything at one time.
BTW, the rad. shop will pressure test for you also. That makes sure there are no leaks.
Be sure the water they use to check with is all dried out before filling......
Al