I have been trying (with little success) to interpolate what Vizard wrote for the Mini with its siamese 2-3 port and individual 1 & 4 ports in a "long center branch" header, but the separation time in degrees looks like too big to make sense of it.
I've wasted many an hour looking for 4-port L6 header designs from long ago, Ford Zodiac for example.
I've seen pictures of a forming tool made from a solid steel bar, shaped on a grinder to match the non-circular port shape (like stovebolt). Not enough text to tell for sure, but looks like:
1. cut a piece of your primary size to 6" length
2. insert (at least) the next smaller size pipe or sand inside to stiffen it
3. hold in padded vise or mandrel
4. grease the pipe ID and tool OD and apply BFH to convert the circle to a rounded square or rectangle.
This works best if the tube is slightly larger than the port ID, since the circumference can only change so much (stretching) before it cracks.
