I would call them about welding on the weight. If it voids warranty, then have them send you the correct one (if they know what that is). As mentioned, all factory 360's are externally balanced. If they don't know this, I'd shop elsewhere.
I just asked them if it would void the warranty welding a weight onto the tc and they said it's fine... The tc I'm taking the weight off is worth less than the flex plate I'd have to buy anyway.
