I would check out Spd exhaust. I build custom headers in a professional shop we get all our header building material form spd they might be a little more expensive than others but they use high quality steel for all there bends and there bend are done on a very good machine which leaves the tube very round after bending which is a huge factor in fitting tubs together once they have been cut and realigned if the cross-sections are not round it will give even a experienced guy a hard time. Spd makes the little piece that you weld between that four tubes now they call it an (fc) star if you look in the formed collectors section of there catalog you will find it. We build our ls header 1 ¾ primary’s stepped to 1 7/8 with 3 inch collectors.
http://www.spdexhaust.com/