Woohoo, getting close to done now. We are on to efficient collector shaping.
A collector is designed to bring tubes together into one larger tube. It should do this smoothly and efficiently to maintain exhaust velocity. The main problem of doing this is shown above. There is a gap between the pipes as they come together, so we need the collector to smooth the transition.
Ideally, you would order this freakin fancy
Burns Stainless collector and call it done. However, they retail for $200 to $400, and the materials for this job are likely going to be under $100... So, lets talk about making your own!
You can try to make your own fancy collector like the one above. This is totally doable, but it really looks like it would take a ton of time. The alternative is you can use this slightly less efficient, but vastly easier to make collector.
This is Headers by Ed's suggestion. You can form the ends of the pipes together without a huge amount of hassle, then you weld the seams together.
Above is a collector I made for a header I started building a long time ago for a Toyota Tercel. I took a bunch of tubes the length I wanted, cut a big V in them, and then started forming the outlet diameter with a hammer. You can calculate the size of the V by figuring out the circumference of your inlet and outlet diameters. It takes a while to form it all, but as you can see its quite doable with just a hammer and vise.
This collector is fairly easy to weld up to the above seam welded pipes without having to worry too much about sealing things perfectly.