Exhaust backpressure

In an attempt to finger out why my junk is overheating, i took it and had the cooling system tested for emissions(smog machine).It had a whopping 2 P.P.M. spike.
Needless to say, that my heat gasket isnt the issue.
I am looking for my gauge so that i can test the exhaust system BEFORE i start cutting and busting up the cat conv.Anyone know what kind numbers i should be looking at when i use the O2 bung?

89
closed system(save your breath about the conversion)
auto
new hoses
fresh coolant
180 t stat (1month old)
water pump less that a year old
radiator is a year and a half old
electric fan is working like it should
mech fan is still tight
still red lines if you drive it more that 55 mph for more than 15-20 minutes

Head gasket leaks can be hard to troubleshoot, mine passed the compression test, passed the leakdown test. Took some fairly clean pieces of an old sheet and shoved some strips down into the cylinders and put 25 PSI to the radaitor, got some green drops of coolant on the white material. Did have some minor bubbles in the cooling system (not noticeably excessive), coolant never did smell wierd. I kind of knew I had a head gasket leak, but was gonna nail it down for sure, before I tore into the head. Did notice my coolant was kind of cloudy, not rust red, kind of a grey tint, along with the normal coolant tint.
Had a guy try to give me a water pump as a replacement, that the impeller was a mirror image of mine. Just luck I had my old water pump with me, for comparision. Contrary to what you´d think, the impeller turned backwords, doesn´t reverse the flow of the coolant, it just pumps the correct direction very poorly.
The lower radiator hose needs the spring inside, so it doesn´t collapse at higher RPM´s. Mine once slipped (it was a touch short) far enough to allow the lower hose to partially collapse and restrict the flow.
The thermostat needs a gasket that isn´t perfectly round. The gasket needs a cutout for the heater surge tank hose. Many universal type gaskets don´t have the cutout. The surge tank hose bypasses the themrostat, so there is some flow, even when the thermostat is closed. This allows the heated coolant to work it´s way (rather quickly) up to the feeler on the thermostat. It´s possible to get some raht er erratic thermostat operation without the surge tank/heater hose cutout in the gasket.
A really lean running motor will overheat, check your plug color. Bone white plugs have been rare on my XJ´s, unless I´ve had an intake gasket leak. The white plugs will tell you pretty close, where it´s leaking, usually near the middle someplace. Intake gasket leaks have sooted up my plugs (rich) at lower RPM´s and turned them white (lean) during long high speed runs.
I´ve taken my CAT off, bounced it on the ground (lightly) a few times and checked to see what comes out, shook it real good, to check if it had clumps inside and looked through it with a flashlight (after I held it up to the sun and got a face full of soot :dunce you can see through mine, pretty well, with a light.
Just a few oddball things I´ve run across.