I wasnt sure from the OP if this was meant to go outside or inside.If its outside...weld-on will do great.
Inside...i dont know, never heard of weld on being used inside and wet.
What threw me off was you wanted to know if it would be watertight. Whywould it matter if its outside the tank? its not holding water is it?
I could see if you had a three sided overflow and wanted to use the tanks back wall as the 4th. I guess I'm just totally confused from all sides. Sorry.