Well, I figured out the stud/ nut size question, and I'll post the specs below for others who might be curious. I also found the exhaust leaks. Plural. Both sides. Apparently, someone in the past decided to use a cutting torch/welder/unknown device to blow two 3/4" holes in the top of the pipe about 3 inches or so downstream of the rear O2 sensors. Holes neatly melted through the pipe, nice and symmetrical, obviously intentional. I'm guessing to give the car a little "rumble". Well, it sounds like a huge exhaust leak to my ears, so I guess I now have an excellent excuse to go buy some exhaust goodies from Mandrel Exhaust Systems....
I (temporarily) plugged the holes as a test, and the 3rd cat doesn't seem to be plugged up, which is the only reason I would do something like THAT, and only
as a temporary fix until I could deal with a plugged 3rd cat problem CORRECTLY. Arrgh.
Anyways, about the stud sizes...
Went to Checker/Schucks/Kragens/O'Reilly's, and found this kit.
It's Dorman Help! Products number 03131, exhaust flange hardware kit.
The stud sizes (in the kit) are M10-1.5 x 72mm, and I've already gotten the bolts loosened up without stripping/breaking/rounding off anything. Took me all afternoon, too. So, tomorrow, I go put in new studs and two exhaust clamp patch thingies to cover those bloody holes. That should tidy things up until the 'Bird gets an exhaust makeover in the next two or three months