You don't weld pipe to the collectors, you buy mid pipes when you buy the long tubes and they get clamped to the header collectors and connect to your stock exhaust. You'll need a tune and o2 sensor extenders as well.
Kooks, Stainless Works and I am sure there are other brands I am not thinking too hard right now.
I gad such a runaround with kooks. They want you to sign a form stating you will never register your car on any road along with provide your vin number to them. I have a 392 but same deal. Give Jessica a call at Stainless Works. Got mine in and they are beautiful. They are being coated right now
Mine came w o2 extenders btw highflow cats as well