Perhaps an offset plate would do it.
I would enlareg the big hole some bit make and make a adapterplate to go in between the body and the mounting plate of the rad.
You need to make 1 or 2 holes extra but I think its far less work (and reversible) than the welding job that you're on.....
As you have a spare panel you could indeed weld it for best visiible result.
I'm not an experienced welder (is that correct english?) but would take a different welding line.