Thanks, guys. I brushed off the carbon (what it appeared to be), so if it is leakage that occurred since reinstalling the pipe, I'll know. There was so much crap from the oil leak this could have been residual I missed cleaning up. I did install the header to the engine exhaust port before tightening up the clamp that joins the left header to the rest of the exhaust system, making sure the header was square. I have another gasket, so perhaps it's worth taking one more pass at it before hitting the road this weekend.
Bob