Alles, what’s up?
321 is, DIN- X12CrNiTi 18 9, or ISO 683/XIII Type 15. It is universally used for high performance Turbo manifolds, and has proven its worth, viability and longevity. Also you want steel that can “glow red” and not break down over the long term, easy to work, and very available, again 321 fills the bill. 310, DIN- X12CrNi 25 20, is better in scaling at higher temperatures, but too ductile.
So I would use 321 because it is a known quantity. With your application, if you wrap your manifold and your adapter, it will glow red under the wrap, plain “steel” would never withstand this environment.
I am familiar with the chrome moly, 13CrMo4 steel, but 321 beats its heat resistance. I frankly would not worry about the expansion rate, 321 has been used for years with no spectacular failures I am aware of.
Whatever you decide, make sure your Merge Collector/Adapter has as little volume as possible, you want the exhaust flow high and the gas concentrated at this point, (obviously not to the point of restriction), but the less expansion allowed, (less volume) the more velocity and the more “agitated” the gas molecules, carrying their waste energy to the turbine.
I still think you should build a support for the turbo, and please don’t use a piece of galvanized electrical tubing (EMT) that has been beaten with a hammer flat on both ends and then had a couple of holes drilled in it, “Mein Großvater hatte das Luftfahrt-Maschinist würde entsetzt sein“. I mean really
The cast adapter, as long as it is using the right steel will be fine, the thickness of the walls will cetainly contain the gases easily, but again a support on the Turbo might help with the longevity.
I really am not too sure about that gasket matching, it seems as if the exhaust gaskets folks are getting now are wayyy oversize, it might be the photos, but I would not hogolicious the head to a size that overshadows the valve diameter. Better to make a set of gaskets, in fact, maybe I will project this and check gaskets from different suppliers, just to see.
Tell us about the cam if you could, lift, duration etc. vs stock.
As far as the valve seat, I am trying to envision where that angle would be, unless you mean the back side of the valve head. If so, this is a very common modification as it removes a huge disturbance to the air flow. In my world the valves come without this „notch“ and swirl polished back side and front. If I want that for my little Mule, I will have to machine it myself.
So wishing you the best
BlueMule