Two O-Rings?? No, no, you want the Macs header for the OS 140, not the hatori header for the OS. The Macs is flanged conventionally. I had a hatori header for the OS 140 and I didn't care for it.
Oops, my bad... 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.
I am thinking of ordering another Macs header as a back-up, which I will need eventually anyway when I get another engine.