Let's hold off on any more welding or installation. Why in the world would a well known, advertised in all the magazines, chassis and suspension company not just tell you to send back the brackets and whatever else..... and send you the correct brackets? Their "fix" idea is WRONG, out of spec, made up, voids the warranty, totally their fault, involves "boxing", can cause unforeseen problems, risk for you, not what you ordered, involves heating up, and welding on, a frame which is not a good idea anytime. If something happens, you will be blamed.
Absolutely the wrong approach here, Am I missing something? A conversation with them you had? DO NOT EXCEPT THIS WORK AROUND.
Their "return policy" says claims must be made in 10 days. They can not expect that you would "catch their error" until you got to the frame bracket phase.
If I am not missing something. email them and tell them you reject there plan, no matter if you accepted it or not. You have been advised that this is not advisable. Do not communicate by phone. You need an email string of events and answers. Get to the boss.
I should have said "New Scrap Metal". In Marks area they would call them remnants. Hoity toity SoCal. Everybody knows old scrap metal ain't good for nothin. New metal is the way to go to avoid serious ramifications and corns on your feet.
