I couldn't really identify where it was coming from which was weird.
One way to pin point the leak is to use a lighter, (I've used one of the long ones good for lighting barbcues) Hold the flame up around the headers and take note if the flame is blown around.
I did poke around with a screw driver at what I thought were gaskets
Hardware stores commonly sell dentist-like tools for picking. These really help.
Are the gaskets obvious when you see them? Or are they covered with black soot?
Covered with soot, not obvious.
2) How often does one need to replace exhaust gaskets?
I can at least speak for Kawasaki; they say everytime you take the darn header off. I never do this though. When I switch to a new header then I install a new set, with the idea this header might seat differently than the last.
Could it be that I overtightened the mounting bolts for the headers?
Hmmm, not likely, but these bolts dont need to be torqued down too hard. Is it the same exhaust can? That would have a larger effect.