For your firing order (1-5-4-2-6-3-7-8), the ones that are 180 degrees apart are 1-4-6-7 and 5-2-3-8; you'll have to cross the middle pipes on one side to the outers on the other. If you really want to get fancy you can also bring them into the collectors so that the pulses rotate around the collector.
You may compromise too much by wanting to build 180 degree headers; to work properly they should be equal length and the right length for the rpm range. I doubt that you'll be able to achieve that unless you can put the collectors over the bellhousing.
Stainless can be thinner.