The temp sensor your lookin for is the last one on the head. (Closest to firewall). It has 2 leads. To check: Use a DVM, cold resistance should be 2.2-2.8 ohms... at 80 degrees C resistance should be 290-370. This according to the book. Now make sure your checking resistance to groud not terminal to terminal!
If you like heres the resistance to the plugs to ground...KEY OFF! Green w/red tracer 4.37 ohms, other one 1.14 ohms. IT'S THE GREEN/RED ONE THAT BREAKS! Usually within 1-2 inches of the plug
The 80C one is a little tuff, it will be different if the engine is hotter... Just checked mine, needle is about 90C and shows 270 ohms, So I guess you can hook it up, watch the meter, when your in range check the temp...Just checked again, I'm at just below 80C meter shows 384 ohms.
Your exhaust has got to be plugged! either the main cat or those pre cats or the muffler. No **** one time I ran into a plugged exhaust problem, relpaced the cat, no change, replaced the muffler, no change, Finally I pulled the whole damn system apart and there it was, the exhaust pipe had a double wall construction, and the inside wall had fallen apart and plugged up the tube! Gezzz, what a mess! Anyway yours is probably the cat. Pep Boys used to sell those "test tubes" a piece of pipe w/ flanges on each end to replace the cat, of course for "test" only. The rusted nuts & bolts are a challenge, just spray em' w/ WD40 a day before let them really soak it up, that will help. I always have new ones ready for install.