So do you have questions? or is this a soon to be DIY thread about youre car.
Ive already done this exact same thing, APS use to make an adapter years ago but has long since stopped making it. They didnt make many, they were cast iron. If you tig weld it all you will be fine. My first mock/test piece cracked with in a couple hundered miles just as i suspected it would do to the heat. Mig welds dont hold up to heat cycling with out the material cracking at the weld seam. Tig welding did the trick and the setup is long gone to a friend over seas so I dont have current pics, not sure i have old ones either because that lap top that had the pics was stolen during a break in at our shop.
I have recently bought more Subaru flanges and I want to make another one for my wifes car soon.
Oh, and you will need 90* pipes to hack up. The final design of the adapter looks like an "S" shape. The 2 90's will need to be cut in half and only use about 4" of each one. The OBX/Garrett rotated kit is pretty much what you need to copy it from just as I did.