I was playing around online and came across the Schoenfeld headers website and it now shows 3 different size headers for LS swapped trucks. There's a 1 3/4" primary stepped to 1 7/8" and a full 1 7/8" primary now offered with along with the standard 1 3/4" which has been out for a while now. Look here:

http://www.schoenfeldheaders.com/LS1-TRUCK.htm