I had excellent success with the OS headers when I switched to Nitrile O-rings. I got them from my local bearing shop. I'm not sure but I think this material handles the fuel and temperatures better.

In any case they've worked for me. Prior to switching over to Nitrile, I found that the standard o-rings kept popping out, splitting etc.

The other thing I do is treat them as disposable items. What I mean by that is that if I adjust the header in any way or remove it , it's time for new o-rings. They seem to last longer when left alone.