I have been making these for a well known race team. These were based on the FIA approved race manifolds. They are stainless steel (304 grade).
The primaries are 38mm, secondaries 44mm and tail pipe 50mm, but I can change the sizes to suit your requirement. These are made to order so please allow 10 to 14 days. I have been building exhaust manifolds and systems for over 20 years. I also make systems for classic cars, race cars, hot rods, American cars, and motorcycles. If you have any questions pleas ask.