I thought you had mentioned somewhere that you would use the OS-brand header made for the OS 1.40. Tower does indeed have such a beast.
Not sure how well the O-rings will stand up to the higher exhaust gas temps in a gas application. They don't last very long on that header when running glow, and I suspect you might be changing them very frequently when running gas. However, if you replace them with Viton O-rings it might work out.
One other thing about the OS header - if I remember right, it's got a fairly restrictive opening to it. Might not work as well as the Macs header in the SAP-180 application.