I commissioned one exhaust manifold from some place once made from 321 stainless, was made to fit HKSGT2 60mm gate and had egt's welded in etc, it was a nice piece short runner like the pics you cant see lol. (but you know what it looks like already) ....... the thing with 321 when you weld it also with 321 filler is its weld pool is "dirty" and the welds sort of come out just like when you use too much heat on 304L if you know what I mean? anyway get it made from 321 and it will last you about 20 engine and turbo oil seals
I tried the long header crap about 10 years ago. Many many people copied me,a and it in short if a con job on rotaries. it just does not work and you have way too many negatives that offset any type of gains associated with running what I call "Horse Cock Manifolds".
My "opinion" is keep it short, keep it divided, keep it direct into the turbo, this will yield massive response, most energy utilization, lowest turbine inlet pressure, most power, any heat losses you have always mean you need to make up the other two of the three things that drive the compressor wheel, FLOW, PRESSURE, HEAT.... Loose any one of these and the other needs to go up to keep the energy equation balanced.
Good luck with it BTW
