I like these guys better than Burns:
Exhaust Components - SPD Home Page B
They offer a 12 degree merge collector which I don't think Burns does. They have everything you need to make 911 headers.
I would not build primaries as short as possible. That will get you a peaky engine which is not usually what you want even for a race car.
After the merge collector, which should have an output smaller than what I see in the photo, you want to transition to the final outlet size. A 7 degree transition is good. The transition gives some restriction which is good.