Thanks to some 11th-hour help from Mr. Scout, I got the second bolt off the exhaust manifold and a new gasket installed, along with two shiny copper bolts of the correct length and diameter. It was tricky work by myself trying to hold a wrench on the top nut while torquing the bottom bolt enough to make things turn; as it worked out, I snapped the bolt in half. Whatever works.
Peer Pressure is quiet again. There’s no backfire coming off a heavy foot in the gas, and exhaust fumes in the cabin are all but gone.