I'm not worried, tank was used for 6 years in the condition shown at beginning of the thread. You cant even tell were the problem was now that the weld on has started setting. I've decided to hold off putting it in the house for at least 48 hours to make sure its set, i have weight on the corner to make sure it seals well.
The original owner is still baffled that I'm doing anything at all. Giving me the "if it ain't broke don't fix it routine".
Gotta remember that it has never at one time in the 6 years it has looked like this in full operation had a leak. And now you cant tell it ever had a problem, so I'm satisfied!
PS my confidence in the work hasn't stopped me from altering my home insurance...
EDIT, I would like to thank you zennzzo for the suggestion. I hope my attitude doesn't come off as cocky/knowitall, I'm just comfortable with the condition of the aquarium. Should anything happen i promise to let you know that you were right!