Since I have been asked about this many times, here are some pics and information for those who want to get into a header fabrication project. Hopefully it will help save some time and frustration.
Here's all the materials it takes to build a set of turbo headers,
About $ 600. of materials, all 308 stainless.
IMO using any material other than stainless is a waste of money.
You need to consider that you are building something that is going to get RED HOT hundreds of times and you still want it to hold good alignment. Any cost savings of mild steel is insignificant IMO as compared to the days, and days of fabrication time it takes to assemble a set of these.
When I first did this in 2003 I blocked the turbos up into place on the engine, in the boat where I felt I could pipe everything into easily and leveled and squared everything as best I could.
I bolted the flanges on both ends and tacked a couple pieces of flatstock between them so that I could remove it and hold that alignment. Then I built a jig to hold the flanges at that alignment. This is an absolute must IMO. Sorry I don't have those old pictures anymore.
I start out with the 4 into 1 collector with a 3 1/2" outlet, saw it down to 6" long, and mash it to a rectangular shape in the vise until it fits the T6 turbo flange. I use 1/2" flange material so that the weld is on the inside of the flange.
Then I bolt the flange down to the jig that I made to hold the flanges in perfect alignment while fitting up the tubes. I have a 3/4" threaded rod coming up through the center of the collector that helps locate the tubes.
Cutting and fitting each tube takes at least an hour per tube. The compound angles make it difficult to get each tube coming out of the flange square and going into the collector square. Also I wanted the end of the tube to be in the center of the flange.
After getting all the tubes tacked up you have to remove some of them and finish weld some of the joints that won't be accessible with everything in place.
After the 4 tubes are tacked up good and inacessible welds are made you need to remove the collector and weld the ends of the tube to each other and cap off the gap between them.
Then I cap off all the ends and hook up the nitrogen gas purge setup before finish welding,
The nitrogen needs to be flowing during the whole finish welding.
Here's a view of the inside showing where the 4 tubes are capped off and the inside weld on the T6 flange,
After everything is welded up you need to locate where the wastegate is going to be.
I used a section of 2 1/2" tube with a Vband flange on the end. I like to weld a 1/8" NPT coupling near the T6 flange to monitor backpressure.
When it's all done you should have something like this,
Heat shields are going to be a necessity also. It doesn't take much, .030 alum does the job fine, you can bend it by hand over the edge of the bench and as you can see from some of my heat shields, ugly, kinky ones will work as well as pretty ones, kind of like some women in that way
I've made many different variations of heat shields through the years.
Good luck to any who want to undertake a project like this, feel free to call me if you want any advise.