As to your spacing question. In my home and what I've seen in others they are minimally spaced about 16" apart which coincidently is the floor joist spacing . And sometimes they are two takeoffs in the same joist bay (one going left, one going right). That said there is one feature of the trunk that somebody knowledgeable maybe designed, and that is that every so often (say 8 - 12') the trunk steps down a size. I beleive this is done to keep up pressure at the takeoffs closer to the furnace. So if you were creating a duct system from scratch a pro designer would be a good thing.