Looking for a temp drop from one side to the other won't help on an SC because it is a top-to-bottom flow, not a cross flow. A Cobra rad is not an option either for an SC. Additionally, its one thing for a cooling system not to be able to keep up with summer in Florida, but he should have no problem in Chicago, and his other SC works fine, so clearly there is something wrong with this car.
If the temp stays at the M, that is fine. If the fan comes on when you unplug the coolant temp sensor, that means that the fan relay is good, the wiring from the sensor to the ECM is good, and the ECM's ability to command the fan on is working. If the engine then is overheating, and the fan does not come on, that tells me that the ECM is not getting the coolant temp signal. This would mean either the sensor is no good, the ECM is no good, or perhaps the sensor for the computer is not immersed in coolant. Did you bleed the cooling system when the water pump was replaced? If not, that would be step one. Once the coolant level is verified, and you know that there aren't any air bubbles in the system, next I would hook up to a scanner of some sort and look at what temp the ECM thinks the coolant is. The gauge and the ECM use different sensors, so just because the gauge reads, doesn't mean that is the temp the ECM thinks it is. I suspect you either have an air bubble, or a bad coolant temp sensor, and that one of those 2 will fix your problem.
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad